Chapter XXII: Part I: Morphology and Physiology (18)

Dewitz has discovered an onward movement of the blood corpuscles,
somewhat independent of the general circulation. This independent
motion of the blood corpuscles is not only a creeping one like the
amœboid motion of the white corpuscles of vertebrates, but they have
besides a peculiar swimming movement. Dewitz noticed this in the
hind wings of a recently emerged meal-worm beetle (_Tenebrio
molitor_), still white and soft, after they had been cut off. The
tissues forming the matrix within the wings constitute a network
filled with blood. The current of blood within the wing thus cut off
may be stopped flowing by a tap on the firmly clamped object-bearer
on which the wing is placed, or by drawing it by an apparatus
described by the same author, to incite in one way or another the
blood corpuscles to swim forwards. When a corpuscle is disposed to
move, we see it first stirring restlessly, or wabbling about, in
this way changing its form; then it moves forwards, and does not
come to a standstill. If it remains still there, after a while, by
tapping, it begins again its movements.

“Should one yet doubt the fact of this spontaneous movement of the
blood corpuscles, he will surely be convinced of its correctness by
observing the so-to-speak reluctantly springing motion of a blood
corpuscle in the wing of _Tenebrio molitor_ with the simultaneous
change of appearance and shape of the corpuscle.”

This spontaneous or independent motion of the blood corpuscles is
also produced by the heating apparatus. As soon as the corpuscles
lie still in the severed wing and they are warmed, the corpuscles
begin to pass through the meshes of the tissue. When cooled, the
motion ceases, but as soon as the temperature rises to a certain
grade, the corpuscles again move onwards.

To explain this independent motion Dewitz thinks that they take up
and then expel the blood-fluid, and in this way cause their motion.
This independent motion is necessitated, in order that the stream of
blood may become so regulated, that the blood corpuscles shall not
be arrested in their course, but even turn back again out of the
farther end of the antennæ and limbs. The chief mechanical power for
the blood circulation must go on independently of the propulsatorial
apparatus and of the heart. (Kolbe.)

LITERATURE ON THE HEART AND ON THE CIRCULATION OF THE BLOOD

_a._ Anatomy of the organs

=Meckel, J. F.= Ueber das Rückengefäss der Insekten. (Meckel’s Archiv,
i, 1815, pp. 469–476.)

=Müller, J. G.= De vasi dorsali Insectorum. Berolini, 1816, pp. 22.

=Serres, P. Marcel de.= Observations sur les usages du vaisseau dorsal
ou sur l’influence que le cœur exerce dans l’organisation des
animaux articulés, etc. (Ann. du Mus. d’hist. nat., 1818, iv, pp.
149–192, 313–380, 2 Pls.; v, 1819, pp. 59–147, 1 Pl.)

=Herold.= Physiologische Untersuchungen über das Rückengefäss der
Insekten. (Schriften d. Gesellsch. z. Beförderung d. Naturk. in
Marburg, 1823, i, pp. 41–107.)

=Carus, C. G.= Entdeckung eines einfachen vom Herzen aus
beschleunigten Blutkreislaufes in den Larven netzilügliger Insekten.
Leipzig, 1827, pp. 40, 3 Taf.

—— Fernere Untersuchungen über Blutlauf in Kerfen. (Acta Acad.
Leopold. Carol., 1831, xv, pp. 1–18, 1 Taf.)

=Stadelmayr, L.= Ansichten vom Blutlauf nebst Beobachtungen über das
Rückengefäss der Insekten. Diss. München, 1829, pp. 24.

=Berthold.= Beitrage zur Anatomie, Zoologie und Physiologie.
Göttingen, 1831.

=Treviranus, G. R.= Ueber das Herz der Insekten, dessen Verbindung mit
den Eierstocken und ein Bauchgefäss der Lepidopteren. (Zeitschr. f.
d. Physiologie, von F. Tiedemann. G. R. u. L. C. Treviranus, 1832,
iv, pp. 181–184, 1 Taf.)

—— Beobachtungen aus der Zootomie und Physiologie. Bremen, 1839.

=Wagner, R.= Beobachtungen über Kreislauf des Blutes und den Bau des
Rückengefässes bei den Insekten. (Isis, 1832, iii, p. 30; vii, pp.
320–331, 778–783, Fig.)

=Bowerbank, J. S.= Observations on the circulation of the blood in
insects. (Ent. Mag., 1833, i, pp. 239–244, 1 Pl; also in Müller’s
Archiv f. Physiolog., 1834, i, pp. 119–120.)

—— Observations on the circulation of the blood and the distribution
of the tracheæ in the wing of _Chrysopa perla_. (Ent. Mag., 1837,
iv, pp. 179–185.)

=Jaeger.= Ueber die Entdeckung von einer Bewegung in den Schuppen des
Schmetterlingsflügel. (Isis, 1837, v, p. 512.)

=Behn, W.= Découverte d’une circulation de fluide nutritif dans les
pattes de plusieurs insectes hémipteres. (Ann. Sc. nat., 1835, Sér.
2, iv, pp. 1–12.)

=Newport, G.= Insecta, in Todd’s Cyclopædia of Anatomy and Physiology,
1839. London, pp. 853–994. On the circulation of the blood, p. 976,
Figs.

=Duvernoy, G. L.= Résumé sur le fluide nourricier, ses réservoirs et
son mouvement dans tout règne animal. (Ann. Sc. nat., 1839, Sér. 2,
xii, pp. 300–346.)

=Dufour, L.= Études anatomiques et physiologiques sur une mouche dans
le but d’eclairer l’histoire des metamorphoses et de la prétendue
circulation dans les insectes. (Ann. Sc. nat. Zool., Sér. 2, 1841,
xvi, pp. 5–14.)

—— Note sur la prétendus circulation dans les insectes. (Compt. rend.
Acad., Paris, 1844, xix, pp. 188–189.)

—— Études anatomiques et physiologiques sur une mouche, dans le but
d’éclaircir l’histoire des metamorphoses et de prétendue circulation
des insectes. (Mém. mathémat. des Savants étrangers, Paris, 1846,
ix, pp. 545–628, 1 Pl.)

—— Sur la circulation dans les insectes. Bordeaux, 1849, 8º, pp. 40.
(Compt. rend. Acad. Sci., Paris, 1849, xxviii, pp. 28–33, 101–104,
163–170.)

—— De la circulation du sang et de la nutrition chez les insectes.
Bordeaux, 1851. (Act. Soc. Linn., Bordeaux, 1851, xvii, p. 9.)

—— Études anatomiques et physiologiques et observations sur les larves
des Libellules, Appareil circulatoire. (Annal. Sci. nat., Sér. 3,
Zool., xvii, 1852, pp. 98–101, 1 Pl.)

=Schröder van der Kolk, J. S. C.= Mémoire sur l’anatomie et
physiologie du _Gastrus equi_. (N. Verhandl. Kl. Nederl. Instit.,
11, 1845, pp. 1–155, 13 Pl.)

=Nicolet, H.= Note sur la circulation du sang chez les Coléoptères.
(Ann. Sc. nat., 1847, Sér. 3, vii, pp. 60–64.)

=Verloren, C.= Mémoire en réponse à la question suivante: éclaircir
par des observations nouvelles le phénomène de la circulation dans
les insectes, en recherchant si on peut la reconnaître dans les
larves de différents ordres de ces animaux. (Mém. couronn. et Mém.
d. savants étrang. de l’Acad. Roy. Belgique, xix, 1847.)

=Blanchard, E.= De la circulation dans les insectes. (Ann. Sc. nat.,
1848, Sér. 3, ix, pp. 359–398, 5 Pls.)

—— Sur la circulation du sang chez les insectes et sur la nutrition.
(Compt. rend. Acad. Sc., Paris, 1849, xxviii, pp. 76–78; 1851,
xxxiii, pp. 367–370.)

—— Nouvelles observations sur la circulation du sang et la nutrition
chez les insectes. (Ibid., pp. 371–376.)

=Joly, N.= Mémoire sur l’existence supposée d’une circulation
péritrachéenne chez les insectes. (Ann. Sc. nat. Zool., Sér. 3,
1849, xii, pp. 306–316.)

=Bassi, C. A.= Rapporto alla sezione di zoologia, anatomia comparata e
fisiologia del congresso di Venezia, sul passagio delle materie
ingerite nel sistema tracheale degli insetti. (Gazette di Milano,
1847, vi; also Ann. Sc. nat. Zool., Sér. 3, 1851, xv., 362–371.)

=Agassiz, Louis.= On the circulation of the fluids in insects.
(Proceed. Amer. Assoc. Adv. Sc., 1849, pp. 140–143; Ann. Sc. nat.
Zool., Sér. 3, xv, 1851, pp. 358–362.)

=Leydig, F.= Anatomisches und Histiologisches über die Larve von
_Corethra plumicornis_. (Zeitschr. f. wissen. Zool., iii, 1851, pp.
435–451.)

=Wedl, C.= Ueber das Herz von _Menopon pallidum_. (Sitzungsber. der k.
Akad. d. Wissensch. Wien., 1855, xvii, pp. 173–180.)

=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Oestriden
Larven. (Sitzungsber. d. k. Akad. d. Wiss. Wien. Math.-naturwiss.
Cl., xli, 1860, pp. 409–496, 2 Taf.; The circulatory system, pp.
463–490.)

=Brauer, Fr.= Beitrag zur Kenntnis des Baues und der Funktion der
Stigmenplatten der Gastrus-Larven. (Verhdl. d. k. k. zool.-bot.
Gesellsch. Wien., xiii, 1863, pp. 133–136.)

=Moseley, H. N.= On the circulation in the wings of _Blatta
orientalis_ and other insects, and on a new method of injecting the
vessels of insects. (Quart. Jour. of Micr. Science, xi, n. s., pp.
389–395, 1871, 1 P1.)

=Graber, V.= Ueber die Blatkörperchen der Insekten. (Sitzber. Akad.
Wien. Math.-naturw. Classe, lxiv, 1871, pp. 9–44, 1 Taf.)

—— Vorläufiger Bericht über den propulsatorischen Apparat der
Insekten. (Sitzber. d. k. Ak. d. Wiss. Wien., lxv, 1872, pp. 16, 1
Taf.)

—— Ueber den propulsatorischen Apparat der Insekten. (Archiv f.
mikroskop. Anatomie, ix, 1873, pp. 129–196, 3 Taf.)

—— Ueber den pulsierenden Bauchsinus der Insekten. (Archiv f.
mikroskop. Anat., xii, 1876, pp. 575–582, 1 Taf.)

=Grobben, Carl.= Über bläschenförmige Sinnesorgane und eine
eigenthümliche Herzbildung der Larve von _Ptychoptera contaminata_
L. (Sitzb. k. Akad. Wissensch. Wien., 1875, lxxii, p. 22, 1 Taf.)

=Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über
den Mechanismus derselben und über die Menge der ausgeatmeten
Kohlensäure. Inaug.-Diss. Chemnitz, 1872, pp. 28.

=Dogiel, John.= Anatomie und Physiologie des Herzens der Larve von
_Corethra plumicornis_. (Mém. Acad. imp. St. Petersbourg, 7 Sér.,
xxiv, 1877, Nr. 10, pp. 37, 2 Pls.) Separate, Leipzig, Voss.

=Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels,
insbesondere der Respiration bei den Insekten. (Reichert’s und Du
Bois-Reymond’s Archiv f. Anatomie u. Physiologie, 1874, pp.
348–361.)

=Béla-Dezso.= Ueber den Zusammenhang des Kreislaufs und der
respiratorischen Organe bei den Arthropoden. (Zool. Anzeiger, i
Jahrg., 1878, p. 274.)

=Plateau, F.= Communication préliminaire sur les mouvements et
l’innervation de l’organe central de la circulation chez les animaux
articulés. (Bull. Acad. roy. de Belgique, Sér. 2, xlvi, 1878, pp.
203–212.)

=Jaworovski, Ant.= Ueber die Entwicklung des Rückengefässes und
speziell der Muskulatur bei Chironomus und einigen anderen Insekten.
(Sitzgsber. d. k. Akad. d. Wissensch. Wien. Math.-naturwiss. Cl.,
lxxx, 1879, pp. 238–258.)

=Zimmermann, O.= Ueber eine eigentümliche Bildung des Rückengefässes
bei einigen Ephemeridenlarven. (Zeitschr. f. wissens. Zool., 1880,
xxxiv, pp. 404–406.)

=Burgess, E.= Note on the aorta in lepidopterous insects. (Proc. Bost.
Soc. Nat. Hist., xxi, 1881, pp. 153–156, Figs.)

—— Contributions to the anatomy of the milk-weed butterfly, _Danais
Archippus_ F. (Anniversary Memoirs Boston Soc. Nat. Hist., 1880, pp.
16, 2 Pls.)

=Vayssière, A.= Recherches sur l’organisation des larves des
Éphémérines. (Ann. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137, 11
Pls.)

=Viallanes, H.= Recherches sur l’histologie des insectes et sur les
phénomènes histologiques qui accompagnent le développement
post-embryonnaire de ces animaux. (Ann. Sc. nat., Sér. 6, xiv, 1882,
pp. 1–348, 18 Pls.)

=Schimkewitsch, W.= Ueber die Identität der Herzbildung bei den wirbel
und wirbellosen Tieren. (Zool. Anzeiger, 1885, viii Jahrg., pp.
37–40, Fig.)

—— Noch etwas über die Identität der Herzbildung bei den Metazoen.
(Zool. Anzeiger, 1885, pp. 384–386.)

=Creutzburg, N.= Ueber den Kreislauf der Ephemerenlarven. (Zool.
Anzeiger, 1885, pp. 246–248.)

=Poletajewa, Olga.= Du cœur des insectes. (Zool. Anzeiger, 1886, ix
Jahrg., pp. 13–15.)

=Selvatico, S.= L’aorta nel corsaletto e nel capo della farfalla del
bombice del gelso. (Padova, 1887, p. 19, 2 Pls.)

—— Die Aorta im Brustkasten und im Kopfe des Schmetterlings von
_Bombyx mori_. (Zool. Anzeiger, 1887, x Jahrg., pp. 562–563.)

=Kowalevsky, A.= Ein Beitrag zur Kenntnis der Excretionsorgane. (Biol.
Centralbl., 1889, ix, pp. 33–47, 65–76, 127–128.)

=Tosi, Alessandro.= Osservazioni sulla valvola del cardias in varii
generi della famiglia delle Apidi. (Ricerche Lab. Anat. R. Univ.
Roma, v, 1895, pp. 5–26, 16 Figs., 3 Pls.)

=Pawlowa, Mary.= Ueber ampullenartige Blutcirculationsorgane im Kopfe
verschiedener Orthopteren. (Zool. Anzeiger, xviii Jahrg., 1895, pp.
7–13, 1 Fig.)

Also the writings of Kolbe.

_b._ The blood, blood corpuscles, leucocytes, and blood tissue

=Wagner, R.= Ueber Blutkörperchen bei Regenwürmern, Blutegeln und
Dipterenlarven. (Müller’s Archiv f. Anatomie u. Physiologie, 1835,
pp. 311–313.)

—— Nachtrage zur vergleichenden Physiologie des Blutes. (Archiv f.
Anat. u. Physiologie, 1838.)

=Newport, G.= On the structure and development of the blood. First
series. The development of the blood corpuscle in insects and other
invertebrata, and its comparison with that of man and the
vertebrata. (Abstr. of the paper in Roy. Soc., 1845, v, pp. 544–546:
also in Ann. Mag. Nat. Hist., Ser. 3, 1845, iii, pp. 364–367.)

=Landois, H.= Beobachtungen über das Blut der Insekten. (Zeitschr. f.
wissens. Zool., xiv, 1864, pp. 55–70, 3 Pl.)

——, =and L. Landois.= Ueber die numerische Entwicklung der
histiologischen Elemente d. Insektenkörpers. (Ibid., xv, 1865, pp.
307–327.)

=Rollett, A.= Zur Kenntnis der Verbreitung des Hämatins. (Sitzgsber.
d. k. Akad. d. Wiss. Wien., lxiv, 1871.)

=Wielowiejski, H. v.= Ueber das Blutgewebe der Insekten. Eine
vorläufige Mitteilung. (Zeitschr. f. wissens. Zool., 1886, xliii,
pp. 512–536.)

=MacMunn, C. A.= Researches on myohæmatin and the histohæmatins.
(Proc. Roy. Soc. London, 1886, xxxix, pp. 248–252.)

=Peyron, J.= Sur l’atmosphere interne des insectes comparée à celle
des feuilles. (Compt. rend. Acad. Sc., Paris, 1886, cii, pp.
1339–1341.)

=Cuénot, L.= Études sur le sang, son rôle et sa formation dans la
série animale. Part 2, invertébrés; note préliminaire. (Arch. Zool.
Expériment., 1888, Sér. 2, v, pp. xliii-xlvii. See also ibid., Sér.
3, 1897, pp. 655, 679–680.)

=Dewitz, H.= Die selbstandige Fortbewegung der Blutkörperchen der
Gliedertiere. (Naturwiss. Rundschau. Braunschweig, 1889, iv Jahrg.,
pp. 221–222.)

—— Eigenthätige Schwimmbewegung der Blutkörperchen der Gliedertiere.
(Zool. Anzeiger, 1889, xii Jahrg., pp. 457–464, Fig.)

=Schäffer, C.= Beitrage zur Histiologie der Insekten. II, Ueber
Blutbildungsherde bei Insektenlarven. (Spengel’s Zool. Jahrbücher
Abt. f. Anat. u. Ontogenie, iii, 1889, pp. 626–636, 1 Taf.)

=Cattaneo, G.= Sulla morfologia delle cellule ameboidi dei Molluschi e
Artropodi. (Boll. Sc. Pavia. Anno 11, 1889, p. 59, 2 Pls.)

=Wagner, W. A.= Ueber die Form der körperlichen Elemente des Blutes
bei Arthropoden, Würmern und Echinodermen. (Biolog. Centralblatt,
1890, x, p. 428.)

=Preyer, W.= Zur Physiologie des Protoplasma. II, Die Funktionen des
Stoffwechsels. Die Saftströmung. (Patanie’s Naturwiss. Wochenschr.,
1891, vi, pp. 1–5.)

=Cholodkowsky, N.= Ueber das Bluten der Cimbiciden-Larven.
(Entomologische Miscellen, vi, Horæ Soc. Ent. St. Petersburg, 1897,
pp. 352–357, 1 Fig. The fluid thrown out through pores or fissures
in the skin is the blood.)

With the writings of Korotaiev, Tichomeroff, Pékarsky, Balbiani,
Korotneff, Cuénot, and others.

_c._ The fat-bodies

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur
plusieurs autres insectes Coléoptères. Du tissu adipeux
splanchnique. (Ann. Sc. nat., viii, 1826, pp. 29–35.)

—— Histoire comparative des métamorphoses et de l’anatomie des
_Cetonia aurata_ et _Dorcus parallelepipedus_. Tissu adipeux
splanchnique. (Ann. Sc. nat., Zoologie, Sér. 2, 1842, xviii, pp.
178–179.)

=Meyer, H.= Ueber die Entwicklung des Fettkörpers, der Tracheen und
der keimbereitenden Geschlechtsteile bei den Lepidopteren.
(Zeitschr. f. wissens. Zool., 1849, i, pp. 175–179, 4 Taf.)

=Fabre, J. H.= Étude sur le rôle du tissu adipeux dans la sécretion
urinaire chez les insectes. (Ann. Sc. nat., Sér. 4, xix, 1862, pp.
351–382.)

=Leydig, Fr.= Einige Worte über Fettkörper der Arthropoden. (Reichert,
u. du Bois-Reymond’s Archiv f. Anat., 1863, pp. 192–203.)

=Lindemann, K.= Zoologische Skizzen. 1. Struktur des Fettkörpers.
(Bull. Soc. Imp. d. Natural. Moscou, 1864, pp. 521–526, 1 Pl.)

=Landois, Leonh.= Ueber die Funktion des Fettkörpers. (Zeitschr. f.
wissens. Zoologie, xv, 1865, pp. 371–372.)

=Wielowiejski, H. v.= Ueber den Fettkörper von _Corethra plumicornis_
und seine Entwicklung. (Zool. Anzeiger, 1883, vi Jahrg., pp.
318–322.)

—— Ueber das Blutgewebe der Insekten. (Zeitschr. f. wissens. Zool.,
1886, xliii, pp. 512–536.)

=Kowalevsky, A. O.= Sur les organes excréteurs chez les arthropodes
terrestres. (Congrès internat. Zool., 2^{me} Sess., pp. 196–205,
Moscou, 1892.)

THE BLOOD TISSUE

Under this name Wielowiejski has included several important tissues or cellular bodies intimately concerned with the nutrition of the insect. These are:—

1. The blood corpuscles. (See p. 407, leucocytes and phagocytes.) 2. The fat-body proper (_Corpus adiposum_). 3. The pericardial fat-body (pericardial cells). 4. The œnocytes. 5. The garland-shaped cord of muscid larvæ. 6. The subœsophageal body, a peculiar organ found by Wheeler in the embryos and young larvæ of Blatta and Xiphidium. 7. The phosphorescent organs.

_a._ The fat-body

In the body cavity of winged insects and of their larvæ occur yellowish masses of large cells filled with small drops of fat, and forming the “fat-body.” It is of various shapes, more or less lobulated or net-like, and covers or envelops parts of the viscera, also forming a layer under the integument (Fig. 143). The tracheal endings are usually enveloped by the fat-body. It is larger in the larvæ than in the adults, especially in Lepidoptera, in them forming a reserve of nutrition, used during metamorphosis and during the formation and ripening of the eggs and male cells.

Wielowiejski has shown that there is a regular arrangement of the
fat-body in the general cavity of the body. For example, in the
larva of Chironomus occur the following forms of this tissue. Around
the periphery, on each side of the body cavity, is a loose network
of lobes with large meshes constituting the peripheral layer or
external lobular fat-body; these lobular masses are segmentally
arranged.

Within these segmental lobes, on each side of and along the
digestive tract, extending along through almost the entire body, is
an unbroken strand of this tissue, forming the internal fat-body
cords. From the first larval stage, and even before hatching, its
cells are so unusually large, being filled with large, clear, mostly
colorless fat-drops, that their limits cannot be defined, and their
nuclei can only with great difficulty be detected. Only in some
large larvæ of Chironomus has Wielowiejski found clearly defined
cells; the protoplasm of these cells contain almost no fat-drops.

The fat-body is of mesodermal origin, and as Wheeler insists, is not
derived from the œnocytes, as supposed by Graber. Formed from the
mesoderm, it is a differentiation of portions of the cœlomic walls,
and therefore metameric in origin. That the fat-body gives origin to
the blood corpuscles Wheeler is doubtful.

The fat-cells are distinct, spherical, and as a rule possess only
one nucleus, though in those of Apis and Melophagus there are two
nuclei, and in Musca several. Sometimes the cells contain a
substance like the white of an egg, and concretions of uric acid, or
these take the place of the fat-drops. The presence of uric acid
shows that a very active metabolism goes on in the fat-body. “In
some cases it has been proved that the fat-body in the larva is rich
in fat and poor in concretions of uric acid, while in the imago it
is poor in fat and rich in concretions of uric acid” (Lang).

Leydig, in 1857 (Lehrbuch der Histiologie), spoke of the presence of
dark concretions in the fat-body, and afterwards (1864) showed that
there was a wide distribution of uric acid salts and concretions.
Witlaczil, also, has detected concretions in the fat-body of the
Psyllidæ, in larval Cecidomyiidæ, in the larvæ and pupæ of ants, and
in the pupa of Musca.

The physiological processes which take place in the fat-bodies are
obscure. Graber regarded the whole system of the fat-bodies as “a
single, many-lobed lung,” while before him Landois, taking into
account the intimate relation existing between the finer tracheal
branches and the fat-body, considered that the latter was concerned
in respiration. Marchal thinks that the fat-body is a urinary organ,
as the urates are formed within the cells of this body.

Moreover, Schäffer maintains that a special kind of fat-body cell
has the important function of taking up and giving out nutritious
matters during the internal processes of metamorphosis, while he
also believes that there is a genetic connection between the
fat-body and the blood corpuscles—a view combated by Wheeler.

Kowalevsky finds that the fat-body remains absolutely insensible to
the action of the substances which stained the Malpighian tubes (p.
352). So long as the cells are healthy and living they are not
stained and do not absorb the colors in question; and this
insensibility persists, even when the cells are of a different
nature, as those of the fly (adipose and “intercalary” cells).

_b._ The pericardial fat-body or pericardial cells

We have already, on p. 405, called attention to these organs, but they also have an intimate relation to the fat-body.

Kowalevsky (1892) remarks that the disposition of these cells varies much in different insects and even in the same animal. Thus, in the Diptera and the ordinary flies there are found around the lower part of the dorsal vessel 13 pairs of large pericardial cells which lie next to a crowded bed of small cells forming a compact mass around the anterior part of the dorsal vessel. In caterpillars, notably silkworms, from the compact layer of pericardial cells which surround the heart, pass off trunks which are directed towards the lateral walls of the body, also forming close networks around the tracheæ and then passing down into the abdominal cavity of the body of the larva.

In the larvæ of certain Hymenoptera, the trunks which pass off from the pericardial region form a loose cord, a sort of fatty tissue covering the entire body cavity.

This tissue, adds Kowalevsky, entirely differs from œnocytes, or
from the so-called glandular body whose formation in Gryllotalpa has
been described by Korotaiev, and in _Bombyx mori_ by Tichomiroff. In
a recent work wherein has been collected everything known regarding
these last-named cells, Pékarsky proves that they are unique in
nature and cannot be regarded either as fat-cells, or as pericardial
cells, or even as formative leucocytes.

As to the structure of the pericardial cells, Kowalevsky adds that they are always attached to muscular fibres passing off from the heart, and that they lie, so to speak, upon them. In the locusts the muscular fibres supporting the pericardial cells appear distinctly like little staves or sticks. The attachment of the pericardial cells to the muscular fibres has been observed by Cuénot and reproduced by him in his work, but his description somewhat differs from that observed by Kowalevsky in the locust (_Acrydium migratorium_).

As to the nature of the acid excretions which are formed in the
pericardial cells, in spite of his attempts to solve the problem,
Kowalevsky has been unsuccessful. The only observations in this
direction are those of Letellier on the pericardial glands of
lamellibranch molluscs, which he found to contain hypouric acid, and
it is probable, says Kowalevsky, that the acidity of the pericardial
cells in insects is due to the presence of the same acid.

=Leucocytes or phagocytes in connection with the pericardial cells.=—It is thought by Schäffer that the leucocytes or phagocytes may be free or wandering fat-body cells. They play an important part in metamorphosis, while they absorb or feed upon the remains of the larval organs, and thus prove of use in the building up of the organs of the adult insects.

While the faculty of _phagocytosis_ is wanting in the urinary tubes, Balbiani and more recently Cuénot have expressed the opinion that the pericardial cells of insects may have the power of absorbing hard bodies, “acting as a phagocytic gland.” This, however, is called in question by Kowalevsky, from studies made on different insects. On introducing powdered carmine into the body of an insect it has not been absorbed by the pericardial cells, as they have not been colored red. It is the leucocytes which absorb the grains of carmine, and which, after having dissolved them, transmit them to the pericardial cells. Hence, then, the pericardial cells have not the phagocytic power of which Cuénot speaks.

Returning to his own observations on hard bodies introduced into insects, or large globules introduced under the form of a milk emulsion, Kowalevsky has found that these bodies were absorbed in the first place by the free-swimming leucocytes, and in the second place by whole groups or nests of leucocytes situated in different parts of the body, principally on the threads of the adipose body. In the Orthoptera the absorption is immediately effected by means of the cells of the membrane which separates the pericardium from the cavity of the body underneath the heart. The regions where the hard bodies are absorbed in great number coincide with the regions of formation of the blood corpuscles. In his researches on the larvæ of Hyponomeuta and other Lepidoptera, Schäffer describes these regions as forming a sort of island. The nests where the blood globules are formed are the most active centres of phagocytosis.

FIG. 384.-Section of the heart (_c_) and pericardial cells (_pc_,
_pc_) from the posterior part of the heart of a fly: _l_, _l_, nests
of leucocytes situated between the heart and pericardial cells.—From
a microphotograph, after Kowalevsky.
]

FIG. 385.—Cross-section of the heart of _Truxalis nasata_ and of the
structures around it: _c_, heart: _ep_, epithelium under the
cuticula (hvpodermis); _or_, ovarian tubes; _pc_, pericardial cells,
with one or two nuclei containing a deposit of carmine; _l_ and
_l′_, group of leucocytes, which have absorbed granules of India
ink.—After Kowalevsky.
]

Balbiani, and also Cuénot, have supposed that the formation of the
blood corpuscles takes place in the pericardial cells, but
Kowalevsky insists that these cells cannot form the leucocytes,
which “are probably formed in different parts of the body, notably
in the special nests [_Herde_ of Jäger] situated near the heart, but
outside of the pericardial cells.”

In Fig. 384, where the nests of leucocytes (_l_) are shown, it is
evident that they are formed where observed, and “could not have
come from the pericardial cells, which have their own structure and
their special function,” these cells being very large and
characteristic.

In Kowalevsky’s preparations of Truxalis, the pericardial cells with
deposits of carmine and the groups of leucocytes (Fig. 385, _l_ and
_l′_) stained with India ink, we have to deal with elements
absolutely different. If the formation of leucocytes was caused by
the pericardial cells, these last would be obliged to free
themselves from their contents and to modify their essential nature.

_c._ The œnocytes

FIG. 386.—Cluster of œnocytes from a nearly mature Phryganeid larva:
_o_, œnocytes; _t_, large tracheal branch; _tt_, smaller tracheal
ramifications; _h_, tracheal hypodermis.
]

FIG. 387.—A nearly mature embryo of Xiphidium ensiferum: _o_, _o_,
œnocyte clusters seen from the surface through the integument; _a_,
pleuropodium of the right side (appendage of the first abdominal
segment); _s_, styli; _c_, cercopods.—This and Fig. 386 after
Wheeler.
]

These cells (Fig. 386), with the exception of the eggs, are the largest in the body, and occur in most if not all winged insects. They were called _œnocytes_ (_oinos_, wine; _kustis_, cyst), by Wielowiejski in allusion to their wine-yellow color. These cells are arranged segmentally (Fig. 387) in clusters, held in place by tracheæ, and are situated mostly on each side of the abdomen, rarely being found in the adjoining parts of the thorax. They are more or less intimately associated with the blood and fat-body. Unlike the fat-body, however, they arise in embryonic life from the ectoderm, either by delamination or by immigration, just behind the tracheal involutions.

The separate cells of each cluster are usually separate, but in rare
cases may fuse in pairs or form smaller clusters. In shape they are
round or oval, often sending out pseudopodia-like processes, by
which they are attached to the tracheal twigs or to each other. “The
cytoplasm, which is very abundant, is full of yellowish granules and
is sometimes radially situated towards its periphery. The large
spherical or oval nucleus contains a densely wound and delicate
chromatic filament.” (Wheeler.)

Graber first pointed out the identity of these clusters of cells
with certain metameric cell-masses in insect embryos, observed by
Tichomiroff in those of the silkworm, and by Korotneff in the embryo
mole-cricket.

Although they resemble the blood corpuscles in some insects, they
are always much larger, and do not seem to be amœboid, while they
are never seen to undergo self-division, or to exhibit any
appearance of giving rise to the blood-cells (Wheeler). They have
not yet been detected in Thysanura (Synaptera) or in Myriopoda.

_d._ The phosphorescent organs

Phosphorescence is not infrequent in the Protozoa, cœlenterates, worms, and has been observed in the bivalve Pholas, in a few abyssal Crustacea, in myriopods (Geophilus), in an ascidian, Pyrosoma, and in certain deep-sea fishes.

FIG. 388.—_A_, sagittal section through the hinder end of a male
Luciola, the organs above the phosphorescent plate only drawn in
outline: _s_, integument of the last segment, somewhat removed by
the section-knife from the phosphorescent tissues; _d_, dorsal layer
of the phosphorescent plate penetrated by irregular tracheal
branches, and rendered opaque by numerous urate concretions imbedded
in it; _v_, ventral phosphorescent layer of the plate, with
perpendicular tracheal stems whose branches, where they pass into
capillaries, bear lumps which stain brown with osmic acid; _n_,
structureless substance (coagulum?) filling the end of the last
ventral segment. _B_, isolated portion of the ventral layer of the
phosphorescent plate; _tr_, tracheal stem surrounded by a
cylindrical lobe: _p_, parenchym cell attached to the cylinder; _c_,
capillary, without the spiral threads; _m_, coagulum stained brown.
_C_, a tracheal stem of the ventral layer: at the fork of the
brown-stained capillaries are lumps stained brown with osmic acid.
_D_, a part of _C_, more highly magnified, showing the remains of
the tracheal end-cells (_tc_) enveloping the brown lumps
(_m_).—After Emery.
]

In insects luminosity is mostly confined to a few Coleoptera, and besides the well-known fireflies, an Indian Buprestid (_Buprestis ocelata_) is said to be phosphorescent; also a telephorid larva. Other luminous insects are the Poduran Anurophorus, Fulgora, certain Diptera (_Culex_, _Chironomus_[60] and _Tyreophora_), and an ant (Orya).

The seat of the light is the intensely luminous areas situated either in the head (Fulgora), in the abdomen (Lampyridæ), or in the thorax (in a few Elateridæ of the genus Pyrophorus). The luminous or photogenic organ is regarded by Wielowiejski and also by Emery as morphologically a specialized portion of the fat-body, being a plate consisting of polygonal cells, situated directly under the integument, and supplied with nerves and fine tracheal branches.

In Luciola as well as in other fireflies, including Pyrophorus, the phosphorescent organ or plate consists, as first stated by Kölliker, of two layers lying one over the other, a dorsal one (Fig. 388, _d_) which is opaque, chalky white, and non-photogenic, and a lower one (_v_), the active photogenic layer, which is transparent. Through the upper or opaque layer and on its dorsal surface extend large tracheæ and their horizontal branches, from which arise numerous very fine branches which pass down perpendicularly into the transparent or photogenic layer of the organ. Each tracheal stem, together with its short branches, is enveloped by a cylindrical mass of transparent tissue, so that only the short terminal branches or very fine tracheal capillaries project on the upper part of the cylinder. These finest tracheal capillaries are not in Luciola filled with air, but with a colorless fluid, as was also found by Wielowiejski and others in Lampyris.

These transparent cylinders, with the tracheæ within, forming longitudinal axes, resemble lobules. These lobules are so distributed that they appear on a surface section of this plate as numerous round areas in which circular periphery the tracheal capillaries are arranged with the axially disposed tracheal end-cells. These “tracheal end-cells” are only membranous enlargements at the base of the tracheal capillaries (Wielowiejski). The cylindrical lobules are separated from each other by a substance consisting of abundant large granular cells (parenchym cells) among which project the tracheal capillaries. The cylindrical lobules extend to the hypodermis and come in contact only by their lateral faces with the parenchym.

The structure of the upper opaque chalky white layer of the phosphorescent organ is, compared with that of the photogenic lower portion, very simple. In its loose, pappose, mass are no cellular elements, but when treated with different reagents it is seen to be filled with countless urate granules (guanine) swimming in the fluid it contains, the cell plasma appearing to be dissolved, the cells having lost their cohesion.

In comparing the phosphorescent plate or organ of Luciola with that of Lampyris, the general structure, including the clear cell elements of the cylindrical lobules, which envelop the perpendicular tracheal twigs and their branches, and also the granular parenchymatous cells are alike in both, though the arrangement and distribution of the elements in Luciola is more regular, in Lampyris the tracheal stems being irregularly scattered through the parenchym.

Wielowiejski found in the larval and female Lampyris a higher degree of differentiation than in the male, and Luciola has a more differentiated photogenic organ than Lampyris, as seen in the more regular structure of the lobules.

As regards the light-apparatus of Pyrophorus, or the cucujo, Heinemann shows that, as in the Lampyridæ, it consists of distinct cells, and may be regarded as a glandular structure. It is rich in tracheæ and the other parts already described. In still later researches on a Brazilian Pyrophorus Wielowiejski shows that the phosphorescent plate consists of two layers, the upper usually being filled with crystalline urate concretions, and entirely like those of the Lampyridæ, consisting of distinct polygonal cells, among which are numerous tracheal stems, with tænidia, coursing in different directions, when freshly filled with air, and sending capillaries into the underlying photogenic layer. The latter shows in its structure a striking difference in the cellular arrangement from that of Lampyrids. In the upper or non-photogenic layer are tracheal capillaries which pass down into the underlying cellular plate and which are in the closest possible relations with the single cells—a point overlooked by Heinemann.

=Physiology of the phosphorescence.=—As is well known, the phosphorescence of animals is a scintillating or glowing light emitted by various forms, the greenish light or luminous appearance thus produced being photogenic, _i.e._ without sensible heat.

Langley rates the light of the firefly at an efficiency of 100 per cent, all its radiations lying within the limits of the visible spectrum. “Langley has shown that while only 2.4 per cent of luminous waves are contained in the radiation of a gas-flame, only 10 per cent in that of the electric arc, and only 35 per cent in that of the sun, the radiation of the firefly (_Pyrophorus noctilucus_) consists wholly of visible wave-frequencies.” (Barker’s Physics, p. 385.)

The spectrum of the light of the cucujo was found by Pasteur to be continuous. (C. R. French Acad. Sc. 1864, ii, p. 509.) A later examination by Aubert and Dubois showed that the spectrum of the light examined by the spectroscope is very beautiful, but destitute of dark bands. When, however, the intensity diminishes, the red and orange disappear, and the green and yellow only remain.

Heinemann studied the cucujo at Vera Cruz, Mexico. At night in a dark room it radiates a pale green light which shows a blue tone to the exclusion of any other light. The more gas or lamp light there is present, the more apparent becomes the yellowish green hue, which in clear daylight changes to an almost pure very light yellow with a very slight mixture of green. “In the morning and evening twilight, more constantly and clearly in the former, the cucujo light, at least to my eyes, is an intensely brilliant yellow with a slight mixture of red. In a dark room lighted with a sodium light the yellow tone entirely disappears; on the other hand, the blue strikingly increases.” As regards the spectrum he found that almost exactly half of the blue end is wanting and that the red part is also a little narrower than in the spectrum of the petroleum flame.

Professor C. A. Young states that the spectrum given by our common firefly (_Photinus?_) is perfectly continuous, without trace of lines either bright or dark. “It extends from a little above Fraunhofer’s line C, in the scarlet, to about F in the blue, gradually fading out at the extremities. It is noticeable that precisely this portion of the spectrum is composed of rays which, while they more powerfully than any others affect the organs of vision, produce hardly any thermal or actinic effect. In other words, very little of the energy expended in the flash of the fire is wasted. It is quite different with our artificial methods of illumination. In the case of an ordinary gaslight the best experiments show that not more than one or two per cent of the radiant energy consists of _visible rays_; the rest is either invisible heat or actinism; that is to say, over 98 per cent of the gas is wasted in producing rays that do not help in making objects visible.”

Panceri also remarks that while in the spectroscope the light of some Chætopteri, Beroë, and Pyrosoma exhibit one broad band like that given by monochromatic light, that of Lampyris and Luciola is polychromatic. (Amer. Nat., vii, 1873, p. 314.)

The filtered rays of Lampyris pass (like Röntgen and uranium rays) through aluminium (Muraoka).

The physiology of insect phosphorescence is thus briefly stated by Lang: “The cells of this luminous organ secrete, under the control of the nervous system, a substance which is burnt during the appearance of the light; this combustion takes place by means of the oxygen conveyed to the cells of the luminous body by the tracheæ, which branch profusely in it and break up into capillaries.”

Emery states that the males of Luciola display their light in two ways. When at night time they are active or flying, the light is given out at short and regular intervals, causing the well-known sparkling or scintillating light. If we catch a flying Luciola or pull apart one resting in the day time, or cut off its hind body, it gives out a tolerably strong light, though not nearly reaching the intensity of the light waves of the sparkling light. In this case the light is constant, yet we notice, especially in the wounded insect, that the phosphorescent plate in its whole extent is not luminous, but glows at different places as if phosphorescent clouds passed over it.

It is self-evident that a microscopic observation of the light of the glow-worm or firefly is not possible, but an animal while giving out its light, or a separated abdomen, may readily be placed under the microscope and observed under tolerably high powers. By making the experiment in a rather dark room Emery saw clear shining rings on a dark background. “All the rings are not equally lighted. Comparing this with the results of anatomical investigation, it is seen that the rings of light correspond with the previously described circular tracheal capillaries, _i.e._ the limits between the tracheal-cell cylinder and the parenchym-cells. The parenchym-cells are never stained of a deep brown; this proves that its plasma may be the seat of the light-producing oxidation. Hence this process of oxidation takes place in the upper surface of the parenchym-cells, but outside of their own substance. The parenchym-cells in reality secrete the luminous matter; this is taken up by the tracheal end-cells and burnt or oxidized by means of the oxygen present in the tracheal capillaries. Such a combustion can only take place when the chitinous membrane of the tracheæ is extraordinarily fine and easily penetrable, as is the case in the capillaries of the photogenic plate; therefore the plasma of the tracheal cells only oxidizes at the forking of the terminal tracheal twigs and in the capillaries.” (Emery.)

The color of the light of Luciola is identical in the two sexes, and the intensity is much the same, though that of the female is more restricted. The rhythm of the flashes of light given out by the male is more rapid, and the flashes briefer, while those of the female are longer, more tremulous, and appear at longer intervals.

Emery then asks: What is the use of this luminosity? Is it only to allure the females of Luciola, which are so much rarer than the males? Contrary to the general view that it is an alluring act, he thinks that phosphorescence is a means of defence, or a warning or danger-signal against insectivorous nocturnal animals. If we dissect or crush a Luciola, it gives out a disagreeable cabbage-like smell, and perhaps this is sufficient to render it inedible to bats or other nocturnal animals. An acrid taste they certainly do not possess.

It has long been known that the eggs of fireflies, both Lampyridæ and Pyrophorus, are luminous. Both Newport and more recently Wielowiejski attributes the luminosity not to the contents of the egg, but to the portions of the fat-body cells or fluid covering on the outside of the eggs, due to ruptures of the parts within the body of the female during oviposition. The larvæ at different ages are also luminous.

The position of the luminous organs changes with age. In the larvæ of Pyrophorus before moulting, according to Dubois, the luminous organs are situated only on the ventral side of the head and prothoracic segment. In larvæ of the second stage there are added three shining spots on each of the first eight abdominal segments, and a single luminous spot on the last segment. These spots are arranged in a linear series and thus form three luminous cords. In the adult beetles there is a luminous spot in the middle of the first abdominal sternite, but the greatest amount of light is produced by the two vesicles on the hinder part of the prothorax, the position of which varies according to the species.

LITERATURE ON PHOSPHORESCENCE

=Peters, W.= Ueber das Leuchten der _Lampyris italica_. (Müller’s
Archiv f. Anatomie, 1841, pp. 229–233.)

=Kölliker, A.= Die Leuchtorgane von Lampyris, eine vorläufige
Mittheilung. (Verhandl. d. phys. medizin. Gesellsch. Würzburg, 1857,
viii, pp. 217–224.)

=Schultze, Max.= Ueber den Bau der Leuchtorgane der Männchen von
_Lampyris splendidula_. (Sitzber. d. niederrhein. Gesellsch. f.
Natur. u. Heilkunde zu Bonn, 1864, Sep. p. 7.)

—— Zur Kenntniss der Leuchtorgane von _Lampyris splendidula_. (Archiv
f. mikroskop. Anat., 1865, i, pp. 124–137, 2 Taf.)

=Wielowiejski, H. Ritter von.= Studien über die Lampyriden. (Zeits. f.
wissens. Zool., 1882, xxxvii, pp. 354–428, 2 Taf.)

=Emery, Carlo.= Untersuchungen über _Luciola italica_ L. (Zeits. f.
wissens. Zool., 1884, xl, pp. 338–355, 1 Taf.)

—— La luce della _Luciola italica_ osservata col microscopio. (Bull.
Soc. Ent. Ital. Anno xvii, 1885, pp. 351–355, 1 Taf.)

=Wheeler, William Morton.= Concerning the blood tissue of the Insecta,
III; Psyche, vi, 1892, p. 255. (Structure of the light organ of
_Photuris pennsylvanica_.)

=Heinemann, C.= Zur Anatomie und Physiologie der Leuchtorgane
Mexikanischer Cucujos, Pyrophorus. (Archiv f. mikroskop. Anat.,
1886, xxvii, pp. 296–383.)

=Dubois, R.= Contribution à l’étude de la production de la lumière par
les êtres vivants. Les Elaterides lumineux. (Bull. Soc. Zool.
France, 1886, Année ii, pp. 1–275, 9 Pls.)

=Wielowiejski, H. Ritter von.= Beiträge zur Kenntniss der Leuchtorgane
der Insekten. (Zool. Anzeiger, 1889, Jahrg. xii, pp. 594–600.)

=Hudson, G. V.= The habits and life-history of the New Zealand
glowworms. (Trans. N. Zealand Inst., xviii, 1890, pp. 43–49, 1 P1.)

=Dubois, R.= Sur le mechanisme de la production de la lumière chez
l’_Orya barbarica_ d’Algérie. (Comptes rend. Acad. Sc. France, 1892,
cxvii, pp. 184–186. Also in Ann. and Mag. Nat. Hist., 1893 (6), xii,
pp. 415–416.)

=Schmidt, P.= Ueber das Leuchten der Zuckmücken (Chironomidæ). (Zool.
Jahrb. Morph., Abth. viii, 1894, pp. 191–216, 2 Taf. Also Ann. and
Mag. Nat. Hist., xv, pp. 133–141, 1895. The light is due to
bacteria. Nature, 1897.)

=Chun, C.= Leuchtorgane und Facettenaugen. (Biol. Centralbl., 1896,
pp. 315–320.)

=Muraoka, H.= (On the filtered rays of Lampyris, Wiedemann’s Annalen,
Dec., 1896.)

See also the writings of Audouin, Carus, D. Turner, Thompson.

THE RESPIRATORY SYSTEM

While land vertebrates breathe by inhaling the air through the mouth into the lungs, insects respire by internal air-tubes (_tracheæ_), which ramify throughout every part of the body and its appendages. The air enters these tubes through a few openings, called spiracles or _stigmata_, arranged segmentally in the sides of the body. These tracheæ are everywhere bathed by the blood, and thus the latter is constantly aërated or kept fresh; the blood not, as in vertebrates or as in molluscs, seeking the lungs or gills, or any specialized respiratory portion of the body where the oxygen combines with the hæmoglobin, but the respiratory tubes, so to speak, themselves seek out the blood and the blood-tissue in every part of the insect body, penetrating to the tips of the antennæ and of the legs, entering the most delicate tissues, even perhaps passing through the walls of epithelial cells. As Lang remarks, the want of an arterial vascular system is compensated for as well as conditioned by the extremely profuse branching of the tracheæ.

FIG. 389.—Rat-tailed larva of Eristalis.
]

The aquatic larvæ of certain dragon-flies (Agrionidæ), may-flies, case-worms, etc., respire by means of tracheal gills or branchiæ, which are either filamental or leaf-like appendages containing tracheæ. Somewhat similar structures appended to the thorax of pupal aquatic Diptera, as in the mosquito and its allies, enable them to breathe while stationed a little beneath the surface of the water. Other larvæ, as the rat-tail larva of Eristalis, etc., lying at the bottom of shallow pools or in ditches, etc., can breathe by raising slightly above the surface a long appendage with two spiracles at the end, through which the air enters the tracheal system. (See p. 461.)

Although Aristotle, as well as the natural philosophers of the Middle Ages, supposed that insects did not breathe, one can easily see that they do by holding a grasshopper or dragon-fly in one’s hand and observing the rhythmical rise and fall of the upper and lower walls of the abdomen, during which the air enters and passes out of the air-openings or spiracles on each side of the body.

It is plain that insects consume very little air, since caterpillars may be confined in very small, almost air-tight tin boxes, and continue to eat and undergo their transformations without suffering from the confinement. According to H. Müller an insect placed in a small, confined space absorbs all the oxygen. Insects can survive for many hours when placed in an exhausted receiver, or in certain irrespirable gases. “Cockroaches in carbonic acid speedily become insensible, but after twelve hours’ exposure to the pure gas they survive and appear none the worse.” (Miall and Denny, p. 165.) Insects of the swiftest flight breathe most rapidly, their great muscular activity requiring the absorption of an abundance of oxygen.

FIG. 390.—Section of Sphinx embryo, showing at _s_ the ectoderm
invaginated, and forming the germ of a stigma and trachea
(_t_).—After Kowalevsky.
]

Warmth, plenty of food, besides muscular activity, increases the demand for oxygen and the quantity of carbonic acid exhaled.

_a._ The tracheæ

FIG. 391.—Portion of a trachea of a caterpillar, with its branches
_B_, _C_, _D_: _a_, peritracheal membrane; _b_, nucleus.—After
Leydig, from Gegenbaur.
]

FIG. 392.—Structure of a trachea, diagrammatic: portions of the
peritracheal membrane (_hy_) and chitinous intima (_cc_) removed to
show the structure; in the chitinous intima or endotrachea (cc) can
be seen the spiral thickenings or tænidia.—After Lang.
]

It will much simplify our conception of the nature of the air-tubes when we learn that they originate in the embryo as tubular ingrowths of the integument (ectoderm), these branching and finally reaching every part of the interior of the body. They are elastic tubes, and being filled with air are silvery in color, though at their origin near the spiracles they are reddish or violet bluish; or, in the larva of Æschna, reddish brown, this tint being due to a finely granular pigment situated in the peritoneal membrane.

FIG. 393.—Longitudinal section of the trachea of _Hydrophilus piceus_:
_ep_, epithelium; _cu_, cuticula; _f_, spiral threads.—After Minot.
]

In their essential structure the tracheæ consist of the chitinous intima, which is a continuation of the cuticle of the integument, and of a cellular membrane or outer layer of cells (a continuation of the hypodermis) called the peritoneal membrane, or ectotrachea (Figs. 392, 393).

Leydig discovered that the spiral filaments are not distinct and separate, but intimately connected with the inner membrane (intima), and he detected the outer or peritoneal membrane, which Chun afterwards found to be epithelial in its nature, Minot stating that it is a true pavement epithelium.

Figure 393 represents a longitudinal section of a large trachea of Hydrophilus, showing the peritoneal membrane (_ectotrachea_, _ep_) and the intima or _endotrachea_, divided into the cuticula (_cu_), with the darker colored inner layer, in which are embedded the dark-colored tænidia (_f_).

FIG. 394.—Testis of Anabrus, showing the ramifications of the
tracheæ.—After Minot.
]

=Distribution of the tracheæ.=—The distribution of the air-tubes, as Lubbock and also Minot state, depends first upon the shape of the organs, and upon the size of those whose size is variable. Around the large, hollow organs (digestive canal, sexual organs) the tracheæ ramify in all directions, forking so that the branches diverge at a wide angle. In the organs which have muscular walls, like the oviduct, the tracheæ run straight when the walls are distended, but have a sinuous course when the walls are contracted. (Minot.)

“Around the organs of more elongated form the branches of the
tracheæ run more longitudinally, as is shown by the air-tubes of the
muscles, which also present some peculiarities worthy of especial
notice.

“A short, thick trunk arrives at the muscular bundle, and dividing
very rapidly, breaks up into a large number of delicate tubes, which
penetrate between the muscular fibres, then terminating in tubes of
exceeding fineness, which at first sight seem to form a network that
might well be called a _rete mirabile_. A closer examination,
however, reveals that it is not a real network, but rather an
interlacing confusing to the eye. The longitudinal direction of the
tracheæ of the muscles presents a striking contrast to the system of
divarication represented in Figs. 13 and 14. The course of the
tracheæ of the Malpighian tubes is also very curious. There is one
large trachea which winds around the tube in a long spiral, giving
off numerous small branches which run to the surface of the tube,
upon which they form delicate ramifications. Each tube has but a
single main trachea, and I think the trachea continues the whole
length of the tube, but of this last point I am not quite sure.”
(Minot.)

While in the nymphs of Orthoptera the tracheæ very closely resemble those of the adult, in larvæ of insects with a complete metamorphosis the tracheæ differ very much in distribution from those of the adult. The larval tracheæ are also more generalized and more like those of the original type than the tracheæ of perfect insects. (Lubbock.)

In general there are two main tracheæ, one passing along each side of the body, near the digestive canal, connected with its mate by a few transverse anastomosing branches, and sending off a branch to each spiracle, this arrangement being most simple and apparent in the maggots of Diptera. From these two main branches smaller twigs branch off into every part of the body with its appendages, passing among the different organs, often serving as cables to hold them loosely in place; they also penetrate into the component parts of the organ themselves, passing into the fat-bodies, and among the fibres of muscles, where they become finely attenuated and refined like the capillaries of the vascular system of vertebrates. (Figs. 395, 396.)

FIG. 395.—_Melanoplus femur-rubrum_, showing distribution of air-tubes
(tracheæ) and air-sacs; _V_, main ventral trachea (only one of the
two shown); _S_, left stigmatal trachea, connecting by vertical
branches with _D_, the left main dorsal trachea; _c_, left cephalic
trachea; _oc_, ocular dilated trachea. From the first, second,
third, and fourth spiracles arise the first four abdominal air-sacs,
which are succeeded by the plexus of three pairs of dilated tracheæ,
I, II, III, in Fig. 396. Numerous air-sacs and tracheæ are
represented in the head and thorax. The two thoracic spiracles are
represented, but not lettered.
]

FIG. 396.—_D_, left dorsal trachea; _S_, left stigmatal trachea; I,
II, III, first, second, and third pairs of abdominal dilated
tracheæ, forming a plexus behind the ovaries; 1, pair of enormous
thoracic air-sacs; 2, pair of smaller air-sacs; 3–7, abdominal
air-sacs; _oc_, ocular dilated trachea and air-sacs; _c_, cephalic
trachea. The relations of the heart to the dorsal tracheæ are
indicated.—Drawn by Emerton from dissections by the author.
]

In the youngest larva of _Corethra plumicornis_ Weismann ascertained
the thickness of the longitudinal stem to be 0.0017 mm. That of the
finest tracheal endings in the silk-glands of the silkworm was found
by Von Wistinghausen to be 0.0016 mm. (Zeits. f. Wiss. Zool. xlix,
1890, p. 575.) Weismann states that in the larvæ of Corethra and
Chironomus the tracheal system is only incompletely developed; the
tracheæ are not united with each other, and in the youngest larvæ
they do not contain air.

FIG. 397.—Tracheal system of the right side of _Machilis maritima_:
_k_, head; I, II, III, thoracic segments; 1–10, abdominal
segments; _s_, stigma.—After Oudemans, from Lang.
]

Each of the two main tracheæ, as Kolbe states, sends off into each
segment of the body three branches.

1. An upper or dorsal branch, which supplies the muscles of the
dorsal region.

2. A middle (visceral) branch, whose twigs pass to the digestive
canal and back to the organs of reproduction.

3. A lower (ventral) branch, whose twigs are distributed to the
ganglia and to the muscles of the ventral region.

In certain Thysanura, as a species of Machilis (Fig. 397), we
probably have the primitive condition of the tracheal system, the
longitudinal and transverse anastomoses being absent, but in other
Thysanura (Japyx, Nicoletia, Lepisma, and a few species of Machilis)
they are present.

As Kolbe remarks, whether the fine ends of the tracheæ are closed or
open, whether after the analogy of the blood capillaries of
vertebrates they anastomose with each other, whether the ends of the
air-tubes pass between the cells or penetrate into them, these
questions are not fully settled. According to Leydig’s[61] latest
views the tracheæ penetrate into the cells and unite with the
hyaloplasma. Hence the process of respiration in the last instance
takes place in the hyaloplasma. This assumption accords with the
fact that in the tracheate Arthropods the terminations of the
tracheæ carry the atmospheric air into the space bounded by the
cellular network, also to the hyaloplasma filling the spaces.
Leydig[62] also thinks that the finest tracheal endings penetrate
into the muscular tissue and unite with the primitive muscular
fibres.