Chapter XII: Part I: Morphology and Physiology (8)

=Saunders, Edward.= Further notes on the terminal segments of aculeate
Hymenoptera. (Trans. Entom. Soc. London, 1884, pp. 251–267, 1 Pl.)

=Palmén, J. A.= Ueber paarige Ausführungsgange der Geschlechtsorgane
bei Insekten. (Helsingfors, 1884, 5 Taf.)

=Haase, E.= Die Abdominalanhänge der Insekten mit Berücksichtigung der
Myriopoden. (Morpholog. Jahrbuch, 1889, xv, pp. 331–435, 2 Taf.)

—— Abdominalanhänge bei Hexapoden. (Sitzungsber. d. Gesellsch.
naturforsch. Freunde, 1889, pp. 19–29.)

=Wheeler, William M.= On the appendages of the first abdominal segment
of embryo insects. (Trans. Wis. Acad. Sc., viii, 1890, pp. 87–140, 3
Pls.)

=Janet, Charles.= Études sur les fourmis, 5^e note, sur la morphologie
du squelette des segments post-thoraciques chez les Myrmicides.
(Mém. Soc. Acad, de l’Oise, xv, pp. 591–611, Figs. 1–5, 1894.)

=Heymons, Richard.= Die Segmentirung des Insektenkörpers. (Anh. Abh.
Akad. Berlin, Phys. Abh., pp. 39, Taf., 1895. See also Die
Embryonalent, von Dermapteren und Orthopteren, under Embryology.)

—— Grundzüge der Entwicklung und des Körperbaues von Odonaten und
Ephemeriden. (Abhandl. k. Preuss. Akad. d. Wissens. Berlin, 1896, 2
Taf., pp. 1–66.)

—— Zur Morphologie der Abdominalanhänge bei den Insecten. (Morphol.
Jahrb., iv, 1896, pp. 178–203, 2 Taf.)

=Peytoureau, S. A.= Contribution à l’étude de la morphologie de
l’armure génital des insectes. (Bordeaux, 1895, pp. 248, 22 Pls., 43
Figs. in text.)

=Verhoeff, C.= Cerci und styli der Tracheaten. (Ent. Nachr., xxi, pp.
166–168, 1895.)

Also the writings of Oudemans, Packard, etc.

_b._ The ovipositor

=Lacaze-Duthiers, Henri.= Recherches sur l’armure génitale femelle des
insectes. (Ann. d. sc. natur., 1849, xii, pp. 353–374, 1 Pl.; 1850,
xiv, pp. 17–52, 1 Pl. (Hyménoptères); 1852, xvii, pp. 206–251, 1 Pl.
(Orthoptères); 1853, xix, pp. 25–88, 4 Pl. (Neuroptères, Thysanures,
Coléoptères, Diptères); pp. 203–237 (Lépidoptères, Aphaniptères en
general. Also separate.))

=Sollmann, A.= Der Bienenstachel. (Zeitschr. f. Wissens. Zool., xiii,
1863, pp. 528–540, 1 Taf.)

=Fengger, H.= Anatomie und Physiologie des Giftapparates bei den
Hymenopteren. (Archiv f. Naturgesch., 1863, pp. 139–178, 1 Taf.)

=Eaton, A. E.= Remarks upon the homologies of the ovipositor. (Trans.
Ent. Soc. London, 1868, pp. 141–144.)

=Packard, A. S.= On the structure of the ovipositor and homologous
parts in the male insects. (Proc. Boston Soc. Nat. Hist., xi, 1868,
pp. 393–399, Figs. 1–11.)

=Lambrecht, A.= Samtliche Teile des Stechapparates in Bienenkörper und
ihre Verwendung zu technischen und vitalen Zwecken. (Bienemwirtsch.
Centralbl., 7 Jahrg., 1871, pp. 5–11.)

=Kraepelin, K.= Untersuchungen über den Bau, Mechanismus und die
Entwicklung des Stachels der bienenartigen Tiere. (Zeitschr. f.
wiss. Zoologie, xxiii, 1873.)

=Dewitz, H.= Vergleichende Untersuchungen über Bau und Entwicklung des
Stachels der Honigbiene und der Legescheide der grünen Heuschrecke.
(Königsberg, 1874.)

—— Ueber Bau und Entwicklung des Stachels und der Legescheide einiger
Hymenopteren und der grünen Heuschrecke. (Zeitschr. f. wissen.
Zoologie, xxv, 1874, pp. 174–200, 2 Taf.)

—— Ueber Bau und Entwicklung des Stachels der Ameisen. (Zeitschr. f.
wiss. Zoologie, 1877, xxviii, pp. 527–556, 1 Taf.)

=Adler, H.= Legeapparat und Eierlegen der Gallwespen. (Deutsche Entom.
Zeitschr., 1877, xxi Jahrg., pp. 305–332, 1 Taf.)

=Cholodkowsky, N.= Ueber den Hummelstachel und seine Bedeutung für die
Systematik. (Zool. Anzeiger, vii, 1884, pp. 312–316.)

=Ihering, H. von.= Der Stachel der Meliponen. (Ent. Nachr., 1886, xii,
Jahrg., pp. 177–188, 1 Taf.)

=Meinert, F.= Bidrag til de danske Myrers Naturhistorie. (Kjöbenhavn,
1890, 68 s.u. Danske Vidensk. Selsk. Skrifter, 5 Raek, v, 3 Pls.)

=Beyer, O. W.= Der Giftapparat von _Formica rufa_ ein reduziertes
Organ. (Jena. Zeitschr. Naturw., 1890, xxv, pp. 26–112, 2 Taf.)

=Carlet, G.= Mémoire sur le venin et l’aiguillon de l’abeille. (Ann.
d. sc. nat. Zool., 7 sér., ix, 1890, pp. 1–17, 1 Pl.)

=Künckel d’Herculais, J.= Méchanisme physiologique de la ponte chez
les insectes orthoptères de la famille des Acridides.—Rôle de l’air
comme agent mécanique et fonctions multiples de l’armure génitale.
(Compt. Rend., cxix, pp. 244–247, 1894.)

Also the writings of Verhoeff, Heymons.

_c._ The external genital armature

=Klug, Johann C. F.= Versuch einer Berichtigung der Fabriciusschen
Gattungen Scolia u. Tiphia. (Ueber u. Mohr Beiträge zur Naturkunde,
i, pp. 8–40, 1805, 1 Taf.)

=Audouin, J. V.=, and =Lachat=. Observations sur les organes
copulateurs males des Bourdons. (Annal. général, d. sc. phys., 1821,
viii, pp. 285–289.)

=Audouin, J. V.= Lettre sur la génération des insectes. (Ann. des sc.
nat., sér. 1, ii, 1824.)

=Suckow, F. W.= L. Geschlechtsorgane der Insekten. (Heusinger,
Zeitschr. organ. Physik., 1828, ii, pp. 231–264, 1 Taf.)

=Rathke, H.= De Libellularum partibus genitalibus. Regiomonti, 1832,
pp. 6 + 38, 3 Pls.

=Siebold, C. Th. E. von.= Ueber die Fortpflanzungsweise der
Libellulinen. (Germar’s Zeitschr. f. Ent., 1840, ii, pp. 421–438.)

=Selys-Longchamps, E. de.= Monographie des Libellulidees d’Europe. 4
Pls., 1840.

=Bassi, C. A.= Studi sulle funzioni degli organi genitali degl’
insetti da lui osservati piu specialmente nella _Bombyx mori_. (Atti
della 5 Riun. d. Scienz. Ital. Lucca, 1844, pp. 39–94.)

=Stein, F.= Vergleichende Anatomie und Physiologie der Insekten, i.
Die weiblich. Geschlechtsorgane der Käfer. Berlin, 1847, 9 Taf.

=Ormancey, P.= Recherches sur l’étui penial considéré comme limite de
l’espèce dans les coléoptères. (Ann. sc. nat., 1849, 3 sér., Zool.,
xii, pp. 227–242.)

=Roussel, C.= Recherches sur les organes génitaux des insectes
coléoptères de la famille des Scarabéides. (Compt. rend. Acad. d.
sc. Paris, 1860, l, pp. 158–161.)

=MacLachlan, R.= A monographic revision and synopsis of the
Trichoptera of the European fauna. London, 1874–80, 59 Pls.

—— On the sexual apparatus of the male Acentropus. (Trans. Ent. Soc.
London, 1872, pp. 157–162.)

=Thomson, C. G.= Nagra anmarkningar ofver arterna af slagtet Carabus.
(Thomson’s Opuscula Entomologica, vii, 1857, pp. 615–729, 1 Pl.)

=Dufour, L.= Sur l’appareil génital male du _Coræbus bifasciatus_.
(Thomson, Archiv ent., 1857, i, pp. 378–381.)

=White, F. Buchanan.= On the male genital armature in the Rhopalocera.
(Trans. Linn. Soc., ser. 1, Zool., i, pp. 357–369, 1876, 3 Pls.)

=Graber, V.= Die Aehnlichkeit im Baue der ausseren weiblichen
Geschlechtsorgane bei den Lokustiden und Akridiern auf Grund ihrer
Entwicklungsgeschichte. (Sitzber. k. Akad. d. Wissensch. Wien.,
1870, lxi, pp. 1–20, 1 Taf.)

=Scudder, Samuel H., and Edward Burgess.= On asymmetry in the
appendages of hexapod insects, especially as illustrated in the
lepidopterous genus Nisoniades. (Proc. Boston Soc. Nat. Hist., 1871,
xiii, pp. 282–306.)

=Chadima, J.= Ueber die Homologie zwischen den männlichen und
weiblichen ausseren Sexualorganen der Orthoptera Saltatoria Latr.
(Mitteil. d. naturwiss. Vereins f. Steiermark, 1872, pp. 25–33, 1
Taf.)

=Hagens, von.= Ueber die Genitalien der männlichen Bienen, besonders
der Gattung Sphecodes. (Berlin Ent. Zeitschr., 1874, pp. 25–43.)

—— Ueber die männlichen Genitalien der Bienengattung Sphecodes.
(Deutschen Entom. Zeitschr., 1882, pp. 209–228, 2 Taf.)

=Lindenman, C.= Vergleichend-anatomische Untersuchung ueber das
männliche Begattungsglied der Borkenkäfer. (Bull. Soc. Imp. d.
Natural. Moscou, 1875–77.)

=Forel, A.= Der Giftapparat und die Analdrüsen der Ameisen. (Zeitschr.
f. wiss. Zoologie, xxx, suppl., 1878.)

=Kraatz, G.= Ueber die Wichtigkeit der Untersuchung des
männlichen Begattungsgliedes der Käfer für die Systematik und
Artunterscheidung. (Deutschen Entom. Zeitschr., 1881, xxv, pp.
113–126.)

—— Ueber das männliche Begattungsglied der europaischen Cetoniiden und
seine Verwendbarkeit für deren scharfe spezifische Unterscheidung.
(Ibid., pp. 129–149.)

=Gosse, Ph. H.= On the clasping-organs ancillary to generation in
certain groups of the Lepidoptera. (Trans. Linn. Soc., 1882, Ser. 2,
Zool., ii, pp. 265–345, 8 Pls.)

—— The prehensors of male butterflies of the genera Ornithoptera and
Papilio. (Proc. Roy. Soc. London, 1881, xxxiii, pp. 23–27.)

=Radoszkowski, O.= Revision des armures copulatrices des males du
genre Bombus. (Bull. Soc. Natur. Moscou, 1884, xlix, pp. 51–92, 4
Pls.)

—— Revision des armures copulatrices des males de la tribu
Philérémides. (Ibid., 1885, lxi, pp. 359–370, 2 Pls.)

—— Revision des armures copulatrices des males de la famille des
Mutillidæ. (Horæ Soc. Ent. Ross., 1885, xix, pp. 3–49, 9 Pls.)

—— Revision des armures copulatrices des males de la tribu des
Chrysides. (Horæ Soc. Ent. Ross, xxiii, 1890, pp. 3–40, 6 Pls.)

=Hofmann, O.= Beiträge zur Kenntnis der Butaliden. (Stett. Ent. Zeit.,
1888, pp. 335–347, 1 Taf.)

=Driedzichi, H.= Revue des espèces européennes du genre Phronia Winn.
(Horæ Soc. Ent. Ross., 1889, xxiii, pp. 404–532, 10 Pls.)

=Sharp, David.= On the structure of the terminal segment in some male
Hemiptera. (Trans. Ent. Soc. London, 1890, pp. 399–427, 3 Pls.)

=Escherich, K.= Die biologische bedeutung der Genitalanhänge der
Insekten. (Verhandl. d. zool. bot. Ges. Wien., 1892.)

—— Anatomische Studien über das männliche Genitalsystem der
Coleopteren. (Zeits. f. wissens. Zool., lvii, pp. 620–641, 1 Taf., 3
figs.)

=Verhoeff, C.= Zur vergleichenden Morphologie der “Abdominalanhänge”
der Coleopteren. (Ent. Nachr., xx, pp. 93–96, 1894. Compare also O.
Schwarz and J. Weise’s criticisms in D. Ent. Zeit., pp. 153–157;
also pp. 101–109, 155–157. Zool. Anzeiger, pp. 100–106, 1894.)

—— Vergleichende—morphologische Untersuchungen ueber das Abdomen
der Endomychiden, etc., und über die Musculature des
Copulationsapparates von Triplax. (Archiv f. Naturg., lxi, pp.
213–287, 2 Taf., 1895.)

—— Vergleichende Untersuchungen über die abdominal Segmente der
weiblichen Hemiptera-Heteroptera und Homoptera. (Verh. Nat. Ver.
Bonn, l, pp. 307–374, 1894.)

=Verhoeff, C.= Beiträge zur vergleichenden Morphologie des Abdomens
der Coccinelliden, etc. (Archiv f. Naturg., lxi, pp. 1–80, 6 figs.,
1895.)

=Boas, J. E. V.= Organe copulateur et accouplement der hanneton.
(Oversigt over det K. Danske Vidensk. Selskab Forhand, 1892,
Copenhagen, 1893, 1 Pl., pp. 239–261.)

=Pérez, J.= De l’organe copulateur mâle des Hyménoptères et de sa
valeur taxonomique. (Ann. Soc. Ent. France, lxiii, pp. 74–81, Figs.,
1894.)

=Goddard, Martha Freeman.= On the second abdominal segment in a few
Libellulidæ. (Proc. Amer. Philos. Soc., xxxv, pp. 205–212, January
11, 1897, 2 Pls.)

Also the writings of Eaton, Emery, Fischer, Forel, Géhin, Godart,
Hagen, Joly, Koletani, Loew, Meinert, Mik, Nicolet, Osten Sacken,
Pictet, Roussel, Schaeffer (1754), Schaum, Schenk, J. B. Smith,
Thompson, Buchanan-White, Brunner von Wattle-Wyll, Weise, Wyenbergh.

The subject of copulation has been treated by Hoffer, Hartig,
Schiedeknecht, Verhoeff, etc.

THE ARMATURE OF INSECTS: SETÆ, HAIRS, SCALES, TUBERCLES, ETC.

FIG. 208.—Larva of _Dryocampa rubicunda_, stage II.—Bridgham _del._
]

=The cuticula.=—The integument is externally either smooth and shining or variously punctured, granulated, tuberculated, striated, or hairy. In certain orders the skin is clothed with flattened setæ or scales, while many forms, as some caterpillars (Figs. 208, 209), beetles (Fig. 210), etc., are protected by spines, horns, etc., these in adult insects often forming secondary sexual characters, usually being more developed in the males than in the females.

FIG. 209.—Larva of _Hyperchiria io_, on hatching.
]

The cuticula is not always smooth, but is often finely granulated or
even minutely spinulated. On the abdominal segments of Anabrus, as
observed by Minot, the cuticula is armed with microscopic conical
nodules scattered irregularly over it. They do not correspond, he
says, in any way to hairs; for they do not rest over pores, nor did
he see any specially modified cells underlying them. “As far as I
have observed, they are mere local irregularities, each nodule being
apparently supported by some four or six unmodified epidermal
cells.” Minot adds that the whole of the cuticula, except the cones
just described and the hairs, is divided into numerous minute
fields, each of which corresponds to a single cell of the underlying
hypodermis. Each field is bounded by a distinct polygonal outline,
and its surface is either covered by a large number of extremely
minute projecting points, as on the dorsal arch of the segment, or
is smooth, as upon the articular membrane and ventral arch. Upon the
sides of the dorsal arch and upon the spiracular membrane each field
has a projecting spine or sometimes two or even three. (See also pp.
28, 30.)

FIG. 210.—_Phanæus pegasus_, ♂, from Mexico.—After Graber.
]

FIG. 211.—Section of integument of _Datana ministra_: _c_, cuticula;
_hyp_, hypodermis; _p_, outer pigmented nodulated layer.
]

The cuticle of lepidopterous larvæ has also been described and
figured by Minot. In the caterpillars of different groups
investigated by him, the cuticle was found to be rough with
microscopic teeth or spinules, erect or flattened and scale-like,
and either densely crowded or scattered, and affording excellent
generic and specific characters. In the slug-worms (Limacodids) we
have observed that the cuticula is unusually rough, especially on
the spiniferous tubercle of Empretia, Parasa, etc. (Fig. 213, _c_).
The skin of the body between the tubercles is seen to be finely
shagreened, due to the presence of fine teeth, which are more or
less curved and bent, these teeth arising from a very finely
granulated surface (_d_). The cuticle of neuropterous,
trichopterous, and tenthredinid larvæ will probably afford similar
cases. The integument of the larva of Datana is, on the black bands,
rough and nodulated, the irregular nodules being filled with a black
pigment, and forming a layer (_p_) external to the true cuticula
(Fig. 211).

FIG. 212.—Hairs of Datana: _f_, formative hair-cell; _c_, cuticula;
_p_, pigmented layer; _hy_, hypodermis.
]

The integument of many insects contains fine canals passing through the chitinous layers and opening externally in minute pores. Certain of the pore-canals communicate with hollow setæ which sit directly over the pores; other pores form the external openings of dermal glands, but in many cases they are empty or only filled with air, and do not have any hairs connected with them. Each of these pores communicates with a hair-forming hypodermal cell, called by Graber a _trichogen_.

=Setæ= (“hairs” and bristles).—The setæ of insects are, as in worms, processes of the cuticle originating from certain of the hypodermal cells. They arise either from a ring-like pit, or from a minute tubercle, and are usually situated at the outlet of a pore-canal, which connects with an underlying cell of the hypodermis (Fig. 212). They are, then, bristle or hair-like processes arising from the hypodermis. Where the hairs or setæ are rubbed off, their site is indicated by a minute ring like a follicle in the chitinous integument. The cuticular hair, says Leydig, is in its first condition the secretion of the cellular element of the skin, and a thread-like continuation of the cell-body may rise up through the pore-canal into the centre of the hair, remaining there permanently.

While the setæ are usually simple, they are often branched, plumose, or spinulose, as in larval Hemerobiidæ, Anthrenus, and Dermestes, the larvæ of certain coccinellid beetles, notably Epilachna, and of Cassida, the larvæ of arctians, etc., and in bees (Anthophila, Megachile, Osmia, Colletes, Apis, etc.).

The use of these spinulose, plumose, and twisted hairs in the bees is clearly shown by J. B. Smith, who states that as these insects walk over flowers, the pollen grains adhere to the vestiture, “and this also accounts for the fact, probably noticed by every observant fruit-grower, that bees frequently bury themselves completely in the blossoms, or roll over every part of them. Such insects are after pollen, not honey, and by so rolling about, the pollen grains are brought into contact with and adhere to the surface of the insect.” The syrphid flies also pollenize flowers, the pollenizing of chrysanthemums being effected, as Smith states, by _Eristalis tenax_, and he adds that the body vestiture of the syrphids “is often composed of spurred and branched hairs.” (For reference to gathering hairs, see p. 45.)

FIG. 213.—Cuticular spinules of larva of Adoneta: _a_, _b_, _c_, _d_,
different forms; _e_, _e′_, caltrops.
]

Certain remarkable spines occur in limacodid larvæ, notably Empretia and Adoneta. These we have called caltrops spines, from their resemblance to the caltrops formerly used in repelling the attacks of cavalry. They are largely concerned in producing the poisonous and irritating effects resulting from contact with the caterpillars of these moths, and are situated in scattered groups near the end of the tubercles. A group of three is represented at Fig. 213, _e_. They are not firmly embedded in the cuticle, but on the contrary appear to become very easily loosened and detached, and they probably, when brought into contact with the skin of any aggressor, burrow underneath, and are probably in part the cause of the continual itching and annoyance occasioned by these creatures. It will be seen by reference to Fig. 213, _e′_, that the body of the spine is spherical, with one large, elongated, conical spine arising from it, the spherical base being beset with a number of minute, somewhat obtuse spinules.

FIG. 214.—Glandular hairs of caterpillars. _A_, _Dasylophia anguina_:
_a_, of body; _b_, of head; _c_, of prothoracic shield. _B_,
_Ceratosia tricolor_: _a_, on body; _b_, on abdominal legs. _C_,
_Schizura ipomeæ_: _a_, from third thoracic segment; _b_, from larva
stage II; _c_, simple setæ from minute warts.
]

=Glandular hairs and spines.=—In some insects occur fine, minute, hollow setæ from which exude, perhaps through pore-canals of extreme fineness, droplets of a clear watery or plasma-like sticky fluid. The club-shaped tenent hairs of the feet of Collembola, and the hairs fringing the feet of Diptera, are modified glandular hairs. Here they serve to give out a sticky fluid enabling the insect to walk on smooth surfaces; they end in a vesicle-like bulbous expansion, which may contain numerous pore-canals. Those of caterpillars were first noticed by Zeller, and Dimmock has particularly described those of the larvæ of Pterophoridæ. They are either club-shaped, or variously forked at the end (Fig. 214, _B_, _a_). They are usually replaced after the first larval moult by ordinary, simple, solid, pointed setæ, and their use in caterpillars is as yet unknown. Whether these hairs, as seems most probable, arise from a specialized glandular hypodermal cell, or not, has not yet been discovered.

FIG. 215.—_A_, group of setæ arising from a subdorsal tubercle: _cut_,
the cuticle; _hy_, the hypodermis; _sc_, the enlarged and
specialized cells of the hypodermis which secrete the spines
themselves; _pglc_, the nuclei which secrete the venomous fluid
which fills the cavity of the seta (_s_), seen at _p_ in a broken
spine. _B_, a short entire, and a long broken seta (_s-p_); _pgle_,
four poison cells; _p_, the poison in the hollow of the spine.
]

These temporary fine glandular hairs are probably the homologues of the larger true glandular bristles and spines of the later stages of certain lepidopterous larvæ, which are brightly colored and lead an exposed life, living through a large part of the summer. In these structures the bristles or spines are hollow, filled with a poisonous secretion formed in a single large, or several smaller specialized hypodermal cells situated under the base of the spine. In the venomous spines of _Lagoa crispata_ the poisonous fluid in the larger spines (Figs. 215, _C_, 216, _b_) is secreted in several large cells situated at the base of the spine, and this is the usual form. In the finer spines of a large tubercle (Figs. 215, _A_, 216) there appears to be a differentiation of the hypodermal cells into two kinds, the large, basal deep-seated, setigenous cells (216, _sc_) and the poison-secreting nuclei (216, _pglc_) situated nearer the base of the setæ. The spines being filled with poison and breaking into bits in the skin of the hands or neck, cause great irritation and smarting. These nettling or poisonous hairs or spines are especially venomous in the larva of Orgyia, _Empretia stimulea_, _Hyperchiria io_, the larvæ of the saturnians (Fig. 217) and lasiocampids, etc. They rarely occur in insects of other orders, though the skin of Telephorus is said by Leydig to bear glandular hairs.

FIG. 216.—Section of a subdorsal tubercle from a larva in stage 1:
_sc_, the setigenous cells, one for each seta; _pglc_, nuclei by
which the poison is secreted; _s_, seta; _p_, poison in middle of a
broken spine; _cut_, cuticle; _sd_, _tub_, spinulated surface of the
subdorsal tubercle.
]

Leydig states that in the stout bristles of Saturnia there is, as in
the integument of the body, a homogeneous cuticula, under which is
the cellular matrix (hypodermis), and the clear contents
(hyaloplasma) are secreted from the blood. The cell-structure of the
hairs consist, as in the cells of the body, of spongioplasma and
hyaloplasma. Leydig has observed the droplets of the secretion of
the caterpillar of _Saturnia carpini_ oozing through distinctly
observable pores, and states that there are similar openings in the
hairs and scales. Dewitz found easily observable openings at the end
of the hair of a large exotic weevil (Fig. 130).

The advanced nymph of Psylla is also armed with clavate glandular
hairs (Fig. 178).

FIG. 217.—Armature of last four segments of _Callosamia promethea_:
_a_, a dorsal seta; _b_, one showing the poison (_p_) within.
]

The tubercles are outgrowths of the body-walls; they are either
smooth, warty, or spiny, as in many caterpillars. While the armature
of insects is of little morphological importance, it is evidently of
great biological importance, the welfare or even the life of the
insect depending upon it; and it varies in each species of insect,
especially in Diptera, where the position of even a single seta
characterizes the species.

FIG. 218.—Section through an antennal pectination of _Saturnia
carpini_: _a_, hypodermis, formative cells of the hairs (_c_); _d_,
cuticula; _e_, trachea.—After Semper.
]

FIG. 219.—Flattened hairs from the lateral tufts of larva of
_Gastropacha americana_: _A_, three from the lateral tuft of
_Heteropacha rileyana_.
]

The mode of development of the hairs was first described by Semper. In the pectination of the antenna of _Saturnia carpini_ he observed that the hairs arise, like the scales of the wings, from large round formative-cells lying in the cavity, which send out through the hypodermis and cuticle a long slender process which finally becomes the hair (Fig. 218).

Tactile hairs are those setæ arising over nerve cells or nerve terminations and will be discussed under the organs of sense.

FIG. 220.—The same in _G. quercifolia_: _a_, a small hair ending in
two minute processes.
]

=Scales.=—In very rare cases the hairs of caterpillars (Fig. 219) are flattened and scale-like, and this passage in the same insect of cylindrical hairs into flattened scale-like ones, shows that the scales are only modified hairs. Also, as we shall see farther on, Semper has proved that their mode of origin is identical. While true scales are characteristic of Synaptera (Thysanura and Colembola), as well as Lepidoptera and Trichoptera, they also occur in the Psocidæ (Amphientomum), in many Coleoptera (Curculionidæ, Cleridæ, Ptinidæ, Dermestidæ, Byrrhidæ, Scarabæidæ, Elateridæ, and Cerambycidæ), and in the Culicidæ, and a few other Diptera, though they are especially characteristic of the Lepidoptera, not a species of this great order being known to be entirely destitute of them.

FIG. 221.—Flattened and spinulated hairs of tufts of larva of
_Acronycta hastulifera_.
]

FIG. 222.—Scales from dorsal tuft, on second thoracic segment of larva
of _Gastropacha quercifolia_.
]

The scales vary much in shape, but are more or less tile-like, attached to the surface of the body or wing by a short slender pedicel, and are more loosely connected with the integument than the hairs, which are thicker at the base or insertion than beyond.

The markings of the scales, both of Synaptera and Lepidoptera, are very elaborate, consisting of raised lines, ridges, or striæ with transverse ridges between. “The striæ of the transparent scales of Micropteryx are from about 500 to 300 to the millimetre, varying in different species. The opaque scales of Morpho, which show metallic reflections, have about 1400 striæ to the millimetre.” (Kellogg.)

The primary use of scales, as observed by Kellogg, is to protect the body, as seen in Synaptera and Lepidoptera. A nearly as important use is the production of colors and patterns of colors and markings, while in certain butterflies certain scales function as the external openings of dermal scent-glands, and they afford in some cases (as first claimed by Kettelhoit in 1860) generic and specific characters. Spuler has shown that the scales are strengthened by internal chitinous pillars. Burgess has observed in the scales of _Danais plexippus_ that the under surface of the scales is usually smooth, or provided with few and poorly developed ridges, and this has been confirmed by Spuler and by Mayer (Fig. 226).

In the irised and metallic scales the ridges, says Spuler, are not divided into teeth, and they converge at the base to the pedicel and also toward the end of the scale (Micropteryx), or end in a single process beyond the middle (the brass-colored scales of _Plusia chrysitis_).

The arrangement of the scales on the wings is, in the generalized moths, irregular; in the more specialized forms they are arranged in bands forming groups, and in the most specialized Lepidoptera they are more thickly crowded, overlapping each other and inserted in regular rows crossing the wings, these rows either uniting with each other or running parallel. (Spuler.) The scattered irregular arrangement seen in Micropteryx is also characteristic of the Trichoptera and of Amphientomum.

FIG. 223.—Portion of a longitudinal section through one of the young
pupal wings of a summer pupa of _Vanessa antiopa_: _s_, young scale;
_leu. cy._, leucocyte; _mbr. pr._, ground membrane; _prc_,
hypodermis-cells.
]

FIG. 224.—Portion of a longitudinal section through one wall only of
the pupal wing of a specimen slightly older than that of Fig. 223;
_s_, older scale.
]

=Development of the scales.=—The mode of origin of the scales was first worked out by Semper in 1886, who stated that in the wing of the pupal Sphinx and Saturnia they are seen, in sections, to arise from large roundish cells just under the hypodermis and which have a projection which passes out between the hypodermis (his “epidermis”) cells, expanding into a more or less spherical vesicle, the latter being the first indication of the future scale. He observed that the scales are not all formed at once, but arise one after another, so that on one and the same wing the scales are in different stages of development.

FIG. 225.—Portion of a longitudinal section through a pupal wing about
eight days before emergence: _s_, formative scale-cell; upper _s_, a
scale.
]

More recently Schaeffer has stated that the scales and also the hairs are evaginations of greatly enlarged hypodermis cells, and still more complete evidence has been afforded by A. G. Mayer (1896). In the wings of Lepidoptera, about three weeks before the imago emerges, certain of the hypodermis cells, which occur at regular intervals, begin to increase in size and to project slightly above the level of the hypodermis; these are Semper’s “formative cells,” and are destined to secrete the scales. They increase in length, and appear as in Fig. 223. In the next stage observed, the projections are much longer (Fig. 224). The hypodermis is now thrown up into a regular series of ridges, which run across the wing. Each ridge, says Mayer, corresponds in position with a row of formative cells, and each furrow with the interval between two adjacent rows. The scales always project from the tops of these ridges. The ground or basal membrane has not participated in this folding, and the deep processes of the hypodermis (_prc_) that once extended to this membrane have largely disappeared. Figure 225 represents a more advanced stage almost eight days before the emergence of the imago.

The scales are originally filled with protoplasm, which gradually withdraws, leaving behind it little chitinous bars or pillars which serve to bind together the upper and lower surfaces of the scales, and finally the scales become “merely little flattened hollow sacs containing only air.” As Mayer shows (Figs. 226, 227), from the study of scales examined four days before emergence of the butterfly (Danais), “the striations upon the upper surface of the scale are due to a series of parallel longitudinal ridges,” while the under side is usually smooth.

The mode of insertion is seen in Fig. 227. The narrow cylindrical pedicel of the scale is merely, according to Semper, inserted into a minute close-fitting socket, which perforates the wing-membrane, and not into a tube, as Landois supposed. Spuler describes a sort of double sac structure or follicle (_Schuppenbalg)_ which receives the hollow pedicel of the scale. This was originally (1860) observed by F. J. Carl Mayer, but more fully examined by Spuler (Fig. 228) though not detected by A. G. Mayer.

FIG. 226.—Portion of a cross-section through the pupal wing of _Danais
plexippus_, about six days before emergence: _sg_, scale; _cta.al_,
wing-membrane; _cl.frm_, formative cell of the scale; _mbr.pr_,
ground-membrane; _fbr.h′drm_, hypodermal fibres of pupal wings. _A_,
portion of a longitudinal section through the pupal wing, eight or
nine days before emergence; _prc_, processes of young hypodermis
scales.—This and Figs. 223–225 after Mayer.
]

=Spinules, hair-scales, hair-fields, and androconia.=—Besides the scales, fine spinules occur on the thickened veins of the wings of the Blattidæ, where they resemble fir-cones; also in the Perlidæ, in the Trichoptera, and in the more generalized Lepidoptera (Micropterygidæ and Hepialidæ), occur, as indicated by Spuler, delicate chitinous hollow spinules scarcely one-tenth as long as, and more numerous than, the scales, which sometimes form what he calls “Haftfelds,” or holding areas. These spinules have also been noticed by Kellogg, and by myself in Micropteryx; Kellogg, and also Spuler, have observed them in certain Trichoptera (Hydropsyche). These also occur on the veins, and detached ones near large one-jointed hairs, or hair-scales, said by Kellogg to be striated. Kellogg has detected these scale-hairs, as he calls them, in Panorpa.

FIG. 227.—View looking down upon the upper (_i.e._ exposed) surface of
one of the large scales situated on the veins of _Danais plexippus_,
about four days before emergence: _clm_, chitinous pillars found in
scales. _A_, _a_ smaller scale, _a_, _a′_, sections of the scales.
_B_, leucocyte found in the larger scale.—After Mayer.
]

FIG. 228.—Scale-follicles: _A_, of a scale of _Galleria mellonella_:
_r_, neck-ring. _B_, the same of _Polyommatus phlæas_. _C_, the same
of a hair on inner edge of hind wing of _Lycæna alexis_ ♀.—After
Spuler.
]

FIG. 229.—_A_, portion of wing of a caddis-fly (Mystacides). _B_,
enlarged, showing the androconia and hair-scales. _C_, a separate
androconium.—After Kellogg.
]

The “hair-scales” of the phylogenetically older Trichoptera correspond to certain scales of Lepidoptera, especially the Psychidæ (Spuler), variously called “plumules” (Deschamps), “battledore scales,” also certain minute cylindrical hairs. To these scent-scales is applied the term _androconia_. They are found, almost without exception, on the upper side of the fore wings, occurring in limited areas, such as the discal spots, or on folds of the wings. Fritz Müller has shown that they function as scent-scales, and are confined to the males. Kellogg has detected androconia-like scales on the wings of a caddis-fly, _Mystacides punctata_ (Fig. 229).

FIG. 280.—Cross-section of androconia surface on wing of _Thecla
calanus_; _a_, androconia; _gl_, gland of base; _s_, ordinary
scales; _w_, wing in section.—After Thomas.
]

Thomas has proved by sections of the wing of Danais, etc., that the androconia arise from glands situated in a fold of the wing (Fig. 230), and he states that the material elaborated by the local glands, and distributed upon the surface of the wing by the androconia, is that which gives to many of the Lepidoptera their characteristic odor. On comparing these “glands,” it is evident that they are groups of specialized formative cells of Semper (trichogens), which secrete an odorous fluid, issuing perhaps from extremely fine pore-canals at the ends of the androconia. They thus correspond to the glandular hairs, poison-hairs, and spines of caterpillars, the formative cells of which contain either a clear lymph or poison.

LITERATURE

_a._ Hairs, bristles, cleaning spines, calcaria, combs, etc.

=Leydig, Franz.= Zum feineren Bau der Arthropoden. (Müller’s Archiv f.
Anat. und Phys., 1855, pp. 376–480.)

=Fobel, Auguste.= Les fourmis de la Suisse. Bâle, 1874.

=Saunders, Edward.= Remarks on the hairs of some of our British
Hymenoptera. (Trans. Ent. Soc. London, 1878, pp. 169–171.)

=Perez, J.= Notes d’apiculture. (Bull. Soc. d’Apic. de la Gironde,
Bordeaux, 1882.)

=Osten Sacken, C. R. von.= An essay on comparative chætotaxy, or the
arrangement of characteristic bristles of Diptera. (Trans. Ent. Soc.
London, 1884, pp. 497–517.)

—— Preliminary notice of a subdivision of the suborder Orthorrhapha
Brachycera (Diptera) on chætotactic principles. (Berlin Ent.
Zeitschr., 1896, pp. 365–373.)

=Janet, Charles.= Études sur les fourmis. 8^e Note. Sur l’organe de
nettoyage tibio-tarsien de _Myrmica rubra_. (Ann. Soc. Ent. France,
1895, pp. 691–704, 6 Figs.)

See also J. B. Smith’s Economic Entomology, 1896, hairs of bees. Also
the writings of De Geer, Huber, Fenger, Mayr, Forel, Canestrini, and
Berlese (1880); Dahl, Cheshire, etc.

_b._ Glandular and poisonous setæ and spines

=Ratzeburg, J. Th. Ch.= Ueber entomologische Krankheiten. (Stettin
Ent. Zeit., 1846, vii, pp. 35–41.)

=Zeller, P. C.= Revision der Pterophoriden. (Linnæa Ent., vi, pp.
319–416, 1852, at p. 356 speaks of “Drüsenhärchen.”)

=Dimmock, George.= On some glands which open externally on insects.
(Psyche, iii, pp. 387–401, 1882.)

=Goossens, Th.= Des chenilles urticants. (Ann. Soc. Ent. France, 1881,
pp. 231–236.) Des chenilles vésicants. (Ibid., 1886, pp. 461–464.)

=Packard, A. S.= Notes on some points in the external structure and
phylogeny of insects. (Proc. Boston Soc. Nat. Hist., xxv, 1890, pp.
83–114, 2 Pls.)

—— A study of the transformations and anatomy of _Lagoa crispata_, a
bombycine moth. (Proc. Amer. Phil. Soc., xxxii, pp. 275–292, 7 Pls.,
1894.)

=Holmgren, Emil.= Studier öfner hudens och de körtelartade hudorganens
morfologi hos Skandinaviska macrolepidopterlarver. (K. Svenska
Vetenskaps-Akad. Handl., xxvii, pp. 1–83, Stockholm, 1895, 9 Pls.)

Also the writings of Leydig, Keller, Bach, Karsten, Scribner, Riley,
etc.

(See also Literature of repugnatorial glands.)

_c._ Androconia

=Deschamps, Bernard.= Récherches microscopiques sur l’organisation des
ailes der Lépidoptères. (Ann. des Sc. nat. [?], iii, pp. 111–157,
1835.)

=Waufor, T. W.= On certain butterfly scales characteristic of sex.
(London, 1867–68.)

=McIntire, S. J.= Notes on the minute structure of the scales of
certain insects. (London, 1871.)

=Anthony, J.= The markings on the battledore scales of some of the
Lepidoptera. (London, 1872.)

=Scudder, S. H.= Antigeny or sexual dimorphism in butterflies. (Proc.
Amer. Acad. Arts and Sc., xii, 1877, pp. 150–158.) Also Butterflies,
etc. (New York, 1881, pp. 192–206, figs.)

=Müller, Fritz.= A prega costal das Hesperideas. (Archivas do Museo
nac. do Rio de Janeiro, iii, pp. 41–50, 2 Pls., 1878.)

=Thomas, M. B.= The androconia of Lepidoptera. (Amer. Nat., xxvii, pp.
1018–1021, 2 Pls., 1893.)

_d._ Scales

=Leydig, Franz.= Zum feineren Bau der Arthropoden. (Archiv f. Anat.
und Phys., 1855, pp. 376–480, 1 Taf.)

=Semper, Carl.= Beobachtungen über die Bildung der Flügel, Schuppen,
und Haare bei den Lepidopteren. (Zeitschrift f. wissensch. Zoologie,
1857, pp. 326–339, 1 Taf.)

=Mayer, F. T. Karl.= Ueber den Staub der Schmetterlingsflügel.
(Allgem. mediz. Centralzeitung, 1860, pp. 772–774.)

=Landois, H.= Beiträge zur Entwicklungsgeschichte der
Schmetterlingsflügel in der Raupe und Puppe. (Zeitschr. f.
wissensch. Zoologie, xxi, 1871, pp. 305–316, 1 Taf.)

=Weismann, August.= Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp.
98–99.)

=Dimmock, George.= Scales of Coleoptera. (Psyche, iv, pp. 1–11, 23–27,
43–47, 63–71, 1883.)

=Schaeffer, Cäsar.= Beiträge zur Histologie der Insekten. (Zool.
Jahrbücher, Abth. f. Anat. u. Ontog., iii, pp. 611–652, 2 Pls.,
1889.)

=Kellogg, Vernon L.= The taxonomic value of the scales of the
Lepidoptera. (Kansas Univ. Quart., iii, pp. 45–89, figs. 1–17, 9
Taf., 1894.)

=Mayer, Alfred G.= The development of the wing-scales and their
pigment in butterflies and moths. (Bull. Mus. Comp. Zool., xxix.,
1896, pp. 209–236, 7 Pls.)

=Spuler, Arnold.= Beiträge zur Kenntniss des feineren Baues und der
Phylogenie der Flügeltedeckung der Schmetterlings. (Zool. Jahrb.
Abth. f. Anat. u. Ontog., viii, pp. 520–543, 1 Taf., 1895.)

—— Ueber das Vorhandensein von Schuppenbalg bei den Schmetterlingen.
(Biol. Centralblatt, xvi, Sept. 15, 1896, pp. 677–679, 3 figs.)

THE COLORS OF INSECTS

The colors and bright markings of insects, especially those of butterflies, render them the most brilliant and beautiful creatures in existence, rivalling and even excelling the gay hues of our most splendidly colored birds. The subject has been but recently taken up and is in a somewhat crude condition, but the leading features have been roughly sketched out by the work of a few observers from a physical, chemical, and biological point of view.

The colors of insects, as of all other animals, are primarily due to the action of light and air; other factors are, as Hagen observes, heat and cold, moisture and dryness, as recently shown by the experiments on butterflies by Dorfmeister, Weismann, W. H. Edwards, and later observers. They have their seat in the integument. Hagen divides colors into optical and natural.

=Optical colors.=—“These,” says Hagen, “are produced by the interference of light, and are by no means rare among insects, but they are solely optical phenomena. Colors by the interference of light are produced in two different ways: either by thin superposed lamellæ, or by many very fine lines or small impressions in very close juxtaposition.

“1. There must be present at least two superposed lamellæ to produce colors by interference. The naked wings of Diptera, of dragon-flies, and of certain Neuroptera often show beautiful interference colors. The wings of Chrysopa and Agrion show interference colors only for a certain time, viz., as long as the membranes of the wings are soft and not firmly glued together. Afterwards such wings become simply hyaline.

“The scales of Entimus and other Curculionidæ are well known for
their brilliancy, and it is interesting to remark that when dry
scales are examined with the microscope, many are found partly
injured, which give in different places different colors, according
to the number of layers which remain. The elytra of some
Chrysomelina and other beetles with iridescent colors probably
belong to the same category.

“2. When there are scales with many fine lines or small impressions close to each other, we have the second mode of producing colors.

“The fine longitudinal and transversal lines of lepidopterous scales seem to serve admirably well to produce the brilliant effect of color-changing butterflies. But there must be something more present, as most of the scales of Lepidoptera are provided with similarly fine lines, and only comparatively few species change colors. I remark purposely that the lines in the color-changing scales are not in nearer juxtaposition.” (Hagen.)

“The colors of butterflies change mostly from purple to blue,
sometimes to yellow. The splendid violet color at the end of the
wings of _Callosune ione_ is brought out by a combination of the
natural with interference colors. Originally the scales are colored
lake-red; but a blue interference color is mixed with it; hence the
violet hue results. The blue tones, _i.e._ the splendid varying blue
of the Morpho butterflies, Schatz claims, owe their hue less to the
interference of light than to a clouded layer of scales situated
over the dark ground, through which the light becomes reflected on
the same. The scales of the Morphids are in reality brown, as we see
by transmitted light; moreover, only the upper side of the scales
sends off blue reflections—the under side is simply brown. But the
blue scales of Urvilliana are also shining blue beneath; by
transmitted light they appear as if clear yellow. The smaragd-green
scales of Priamus show by transmitted light a bright red-orange, and
the orange-yellow of Crœsus a deep grass-green.” (Schatz in Kolbe.)

“Krukenberg presumes the golden-green color of _Carabus auratus_ to
be an interference color. It is not changed by the interference of
light, nor was he able to extract from the elytra any green pigment
with ether, benzol, carbon of sulphur, chloroform, or alcohol, even
after having previously submitted the elytra to the influence of
muriatic acid or ammonia. Chlorophyll is not present, whether free
or combined with an acid.” (Hagen.)

Leydig has shown that the interference colors of the hairs of
certain worms (Aphrodite and Eunice) may be produced by very small
impressions in juxtaposition, which bring about the same effect as
striæ. Such an arrangement occurs on the feathers of birds, _i.e._
on the necks of pigeons and elsewhere, and Hagen suggests that this
kind of interference colors occurs more frequently among insects
than is commonly known. At least the limbs of certain forms appear
yellow, but when held in a certain position change to brown or
blackish. “I know of no other explanation of this not uncommon fact
on the legs of Diptera, of Hymenoptera, and of Phryganidæ.”
Interference colors, he adds, may occur in the same place together
with natural colors. “The mirror spots of _Saturnia pernyi_ show
besides the interference colors a white substance in the cells of
the matrix, which Leydig believes to be guanin. But this fact is
denied by Krukenberg for the same species and also for _Attacus
mylitta_ and _Plusia chrysitis_.”

=Natural colors.=—These are divided by Hagen into _dermal_ (cuticular) and _hypodermal_. The dermal colors are due to pigment deposited in the form of very small nuclei in the cuticula. Hagen considers them as “produced mostly by oxidation or carbonization, in consequence of a chemical process originating and accompanying the development and the transformations of insects.”

“To a certain extent the dermal colors may have been derived from
hypodermal colors, as the cuticula is secreted by the hypodermis,
and the colors may have been changed by oxidation and air-tight
seclusion. The cuticula is in certain cases entirely colorless,—so
in the green caterpillar of _Sphinx ocellata_; but the intensely red
and black spots of the caterpillar of _Papilio machaon_ belong to
the cuticula, and only the main yellow color of the body to the
hypodermis.” (Leydig, Histiol., p. 114.)

“The dermal colors are red, brown, black, and all intermediate shades, and all metallic colors, blue, green, bronze, copper, silver, and gold. The dermal colors are easily to be recognized as such, because they are persistent, never becoming obliterated or changed after death.” (Hagen.)

Minot and Burgess refer to the cuticular colors of the cotton-worm
(Aletia), the dark brown color belonging to the cuticula or crust.
“Upon the outside of the crust is a very thin but distinct layer,
which in certain parts rises up into a great number of minute,
pointed spines that look like so many dots in a surface view. Each
spine is pigmented diffusely, and together they produce the brown
markings. The spines are clustered in little groups, one group over
each underlying hypodermal cell.” (U. S. Ent. Comm., 4th Report, p.
46.) Minot also shows that in caterpillars generally a part of the
coloration is caused by pigmentation of the cuticula.

In a dull-colored insect, such as the Mormon cricket (Anabrus), the
coloration, as Minot states, depends principally upon the pigment of
the hypodermis shining through the cuticula. “Most of the cells
contain dull, reddish-brown granules, but scattered in among them
are patches of cells bright green in color. I have observed no cells
intermediate in color; on the contrary, the passage is abrupt, a
brown or red cell lying next a green one. Indeed, I have never seen
any microscopic object more bizarre than a piece of the epidermis of
Anabrus spread out and viewed from the surface.” (2d Report U. S.
Ent. Comm., p. 189.)

The pigment may extend through the entire cuticula, but it is usually confined to the outermost layers, and occurs there in union with a peculiar modelling of the upper surface into microscopic figures which are of interest not only from their delicacy, but because they vary with each species. (See p. 184.)

The hypodermal colors, situated in the hypodermis, are, according to Hagen, the result of a chemical process, generating color out of substances contained in the body. They are easily recognized, since they fade, change, and disappear after death. But where these colors are preserved after death and enclosed in air-tight sacs, as in the elytra and scales and hairs of the body, they persist, though, as we well know, they may fade after exposure to light.

The hypodermal colors are mostly brighter and lighter than the dermal ones, being light blue or green in different shades, yellow to orange, and the numerous shades of these colors combined with white; exceptionally they are metallic, as in Cassida, and are then obliterated after death.

“The fact that such metallic colors can be retained in dead
specimens by putting a drop of glycerine under the elytra, leads us
to conclude that those colors are based upon fat substances. The
hypodermal colors are never glossy, as far as I know; the dermal
colors frequently.

“As the wings, elytra, and hairs all possess a cuticula, dermal
colors are frequently to be found, together with hypodermal ones,
chiefly in metallic colors. In the same place both colors may be
present, or one of them alone. So we find hypodermal colors in the
elytra of Lampyridæ. In the elytra of the Cicindelidæ the main
metallic color is dermal, the white lines or spots are hypodermal,
by which arrangement the variability in size and shape of those
spots is explained.

“There occur in a number of insects external colors, that is, colors
upon the cuticula, which I consider to be in fact displaced
hypodermal colors: the mealy pale blue or white upon the abdomen of
some Odonata, the white on many Hemiptera, the pale gray on the
elytra and on the thorax of the Goliath beetle, and the yellowish
powder on Lixus. Some of these colors dissolve easily by ether or
melt in heat, and some of them are a kind of wax. I believe that
those colors are produced in the hypodermis, and are exuded through
the pore-canals.” (Hagen.)

The white colors are simply for the most part due to the inclusion of air in scales. The white mother-of-pearl spots of Argynnis are produced by a system of fine transverse pore-canals filled with air; in Hydrometra the white ventral marks have the same origin. (Leydig.)

The further statements and criticisms of Hagen regarding the relation of color to mimicry, sexual selection, and the origin of patterns are of much weight and will be referred to under those heads. Indeed, these subjects cannot well be discussed without reference to the fundamental facts stated in the masterly papers of Leydig and of Hagen, and much of the theorizing of these latter days is ill-founded, because the colors of insects and animals are attributed to natural selection, when they seem really the result of the action of the primary factors of organic evolution, such as changes of light, heat, cold, and chemical processes dependent on the former.

As to the chemical nature of color, Hagen, after quoting the results of Krukenberg and others, thinks that the colors of insects are chemically produced by a combination of fats or fat-acids with other acids or alkalis under the influence of air, light, and heat. He concludes:—

1. That some colors of insects can be changed or obliterated by acids.

2. That two natural colors, madder-lake and indigo, can be produced artificially by the influence of acid on fat-bodies.

3. As protein bodies in insects are changed into fat-bodies, and may be changed by acids contained in insects into fat-acids, the formation of colors in the same manner seems probable.

4. That colors can be changed by different temperatures.

5. That the pattern is originated probably by a combination of oxygen with the integument.

6. That mimicry of the hypodermal colors may be effected by a kind of photographic process.

7. Finally, color and pattern are produced by physiological processes in the interior of the bodies of insects.

Krukenberg concludes that change of color (in perfectly developed
insects) is a consequence of the change of food, and can be
explained by the alteration of the pigment through heat and light.
His experiments were made in order to ascertain the cause of the
turning of green grasshoppers in autumn into yellow and pink. He
tried to answer two questions: First, does the pigment of
grasshoppers originate directly out of the food, and does it consist
of pure chlorophyll or of a substance containing chlorophyll, or is
it to be accepted as a peculiar product of the organism? Second, is
the color the consequence of only one pigment, or of several?
Special analysis proves that the green color has no connection with
chlorophyll. He concludes: “It is evident that the green color of
the grasshopper is the consequence of several different pigments
which can be separated by a chemical process.” Krukenberg believes
that light has a marked influence on the color of insects and that
light turns to red or pink the insects which were green during the
summer. It would seem, however, more probable that cold was the
agent, the change being due to the colder autumn weather.

Here we might refer to the results of the studies of Buckton and
Sorby, on the changes in color of Aphides:—

“1. The purple coloring matter appears to be a quasi-living
principle, and not a product of a subsequent chemical oxidizing
process. Mounted in balsam or other preserving fluids, the darker
species stain the fluid a fine violet.

“2. As autumn approaches and cold weather reduces the activity of
the Aphides, the lively greens and yellows commonly become converted
into ferruginous red, and even dark brown, which last hue in reality
partakes more or less of intense violet or purple. These changes
have some analogy with the brilliant hues assumed by maple and other
leaves during the process of slow decay.

“3. Aqueous solutions of crushed dark brown and yellow-green
varieties of Aphides originate different colors with acids and
alkalies.

“4. In the generality of cases coloring-matters, such as indigo,
Indian yellow, madder-lake, and the like, do not separately exist in
the substance of vegetables, but the pigments are disengaged through
fermentation or oxygenation. Again, alizarin itself is reddish
yellow, but alkaline solutions strike it a rich violet just as we
find them to act towards the substance which Mr. Sorby calls
aphidilutein.

“5. Mr. Sorby’s four stages of the changes effected by the oxidation
of aphideine produce four different substances.”

=Chemical and physical nature of the pigment.=—Researches in this difficult field of inquiry have been made by Landois (1864), Sorby (1871), Meldola (1871), by Krukenberg (1884), and more recently by Coste, Urech, Hopkins, and Mayer, and the subject is of fundamental importance in dealing with mimicry and protective coloration, the primary causes of which appear to be due to the action of physical and chemical agents.

Over twenty years ago Meldola observed that the yellow pigment of the sulphur-yellow butterfly (_Gonopteryx rhamni_) was soluble in water, and showed that its aqueous solution had an acid reaction.

Besides the yellow uranidin found by Krukenberg in different beetles
and lepidopterous pupæ, still other coloring-matters, which are very
constant in different species are readily recognized by the
spectroscope. “Thus there appear in the brownish yellow lymph of
_Attacus pernyi_, _Callosamia promethea_ and _Telea polyphemus_,
after saponification of the precipitated soap readily effected by
ether, or incompletely or not removed by benzine, a chlorophane-like
lipochrome; and in the yellowish green lymph of _Saturnia pyri_ and
of _Platysamia cecropia_ besides this pigment still another whose
spectrum shows a broad band on D, but which disappears with the
addition of acetic acid or ammonia, as also after a long heating of
the lymph up to 66° C.”

Coste, and more especially Urech, have shown that many of the pigments may be dissolved out of the scales by means of chemical reagents, giving colored solutions, and leaving the scales white or colorless. They have also shown that some of these pigments may be changed in color by the action of reagents, and then restored to their original color by other reagents. They have proved that reds, yellows, browns, and blacks are always due to pigments, and in a few cases greens, blues, violets, purples, and whites, and not, as is usually the case, to structural conditions, such as striæ on the scales (Mayer). They confined themselves solely to the chemical side of the problem, not considering the structure of the scales themselves.

Urech has also discovered a beautiful smaragd-green coloring-matter in the wings (not in the scales) of the pupa of _Pieris brassicæ_. It is not chlorophyll, and Urech suggests that it may be either the germinal substance of the pigments of the scales or its bearer. It is not the pigment of the blood.

Urech has also demonstrated that in many Lepidoptera the color of the urine which is voided upon emergence from the chrysalis is similar to the principal color of the scales.

Hopkins has worked on the pigments within the scales of butterflies.
The yellow pigment in _Gonopteryx rhamni_ is a derivation of uric
acid, and he calls it lepidotic acid. Its aqueous solution is
strongly acid to litmus, and must be bad-tasting to birds.

Hopkins has dissolved the red pigment from the border of the hind
wing of _Delias eucharis_, an Indian butterfly, in pure water,
finding as the result a yellow solution; but if the solution be
evaporated to dryness, the solid residue of pigment is red once
more. He has obtained from this pigment of _eucharis_ a silver
compound which contains a percentage of metals exactly equal to that
from the pigment of _G. rhamni_. (Nature, April 2, 1892.)

“The scales of the wings of the white butterflies (Pieridæ) are also
shown by Hopkins to contain uric acid, this substance practically
acting as a white pigment in these insects. A yellow pigment, widely
distributed in the same family, is shown to be a derivative of uric
acid, and its artificial production as a by-product of the
hydrolysis of uric acid is demonstrated. That this yellow pigment is
an ordinary excretory product of the butterfly is indicated by the
fact that an identical substance is voided from the rectum on
emergence from the pupa. These excretory pigments, which have
well-marked reactions, are apparently confined to the Pieridæ, and
are not found in other Rhopalocera. This fact shows that when a
Pierid mimics an insect belonging to another group, the pigments of
the mimicked and mimicking insects, respectively, are chemically
quite distinct. Other pigments existing, not in the scales, but
between the wing-membranes, are shown to be of use for ornament.”
(Proc. Royal Soc., London, 1894.)

Griffiths (1892) claims that the green pigment found in several
species of Papilio, Hesperia, and Limenitis, also in Noctuidæ,
Geometridæ, and Sphingidæ likewise consists of a derivative of uric
acid, which he calls lepidopteric acid. By prolonged boiling in HCl
it is converted into uric acid.

Spuler, however, finds that green does not depend on pigmentation,
but is an optical color. As remarked by Spuler, either the chitin of
the scales itself is colored reddish (yellow grayish), or the
pigment is secreted in the nuclei.