Chapter VIII: Part I: Morphology and Physiology (4)

“Bees have the power, by driving blood into the tongue, of forcing
the rod out from the sheath, and distending the wrinkled membrane so
that in section it appears as at _H_, Fig. 86, the membrane assuming
the form of a pouch, given in full length at _A_. It will be seen at
once that this disposition of parts abolishes the side ducts, but
brings the central duct to the external surface. The object of this
curious capability on the part of the bee is, in my opinion, to
permit of cleaning away any pollen grains, or other impediment that
may collect in the side ducts. The membrane is greasy in nature, and
substances or fluids can be removed from it as easily as water from
polished metal. If, now, the sides of a needle, previously dipped
into clove oil in which rosanilin (magenta) has been dissolved, so
as to stain it strongly red, be touched on the centre of the rod,
the oil immediately enters, and passes rapidly upwards and
downwards, filling the trough.”

=Does the hypopharynx represent a distinct segment?=—The facts which
suggest that the hypopharynx may possibly represent a highly
modified pair of appendages, arising from a distinct intermaxillary
segment, are these: Heymons plainly shows that, in the embryo of
Lepisma, the hypopharynx originates as a transverse segment-like
fold in front of the 2d maxillary segment, and larger than it, and
though he does not mention it in his text, it appears like the
rudiment of a distinct segment; the hypopharynx of Ephemeridæ;
arises and remains separate in the nymph from the labium (see
Heymons’ Fig. 29, and there are two lateral projections; see also
Fig. 72, and Vayssiere’s view that it may represent a pair of
appendages; Kolbe also regards it as representing a third pair of
maxillæ, his endolabium, p. 213). Though what is called an unpaired
organ, it is composed of, or supported by, two bilaterally
symmetrical styles, both in Myriopods (Fig. 6, labiella, _stil_) and
in insects (Fig. 77, etc.). On the other hand, in the embryo of
pterygote insects, an intermaxillary segment has not been yet
detected.

LITERATURE OF THE MOUTH-PARTS OR BUCCAL APPENDAGES

_a._ General

=Savigny, Jules-César.= Mémoires sur les animaux sans vertèbres.
1^{re} Part. Description et classification des animaux invertébrés
et articulés, etc. Fasc. 1^{re}. Mém. 1–2. Théorie des organes de la
bouche des crustacés et des insectes. 12 Pl., Paris, 1816, pp.
1–117.

=Gerstfeld, Georg.= Ueber die Mundteile der saugenden Insekten.
Dorpat, 1853.

=Olfers, Ernestus V.= Annotationes ad anatomiam Podurarum. Berolini,
1862, 4 Pls.

=Gerstaecker, Carl Eduard Adolph.= Zur Morphologie der Orthoptera
amphibiotica. (Festschrift zur Feier des hundertjährigen Bestehens
der Gesellschaft naturf. Freunde zu Berlin. 4º, 1873, pp. 39–59, 1
Taf.)

=Muhr, Joseph.= Die Mundteile der Orthoptera. Ein Beitrag zur
vergleichenden Anatomie. (Jahrbuch “Lotos.” Prag, 1877, pp. 40–71, 8
Taf.)

=Burgess, Edward.= The anatomy of the head and the structure of the
maxilla in the Psocidæ. (Proc. Boston Soc. Nat. Hist., xix, 1878,
pp. 291–296, 1 Pl.)

=Meinert, Fr.= Sur la conformation de la tête et sur l’interpretation
des organes buccaux chez les insectes, ainsi que sur la systématique
de cette ordre. (Ent. Tidsskr., 1. Arg., 1880, pp. 147–150.)

—— Tungens udskydelighed hos Steninerne, en slaegt af Staphylinernes
familie. (Vidensk. meddel. fra den naturh. Foren, 1884–1886, pp.
180–207, 2 Pls. Also Zool. Anzeiger, 1887, pp. 136–139.)

=Müller, A.= Vergleichend-anatomische Darstellung der Mundteile der
Insekten. Villach, 1881, 3 Taf.

=Kraepelin, Karl.= Ueber die Mundwerkzeuge der saugenden Insekten.
(Zool. Anzeiger, 1882, pp. 574–579.)

=Dewitz, H.= Ueber die führung an den Körperanhangen der Insekten.
(Berlin. Zeitschr. xxvi., 1882, pp. 51–68, Figs.)

=Wolter, Max.= Die Mundbildung der Orthopteren mit specieller
Berücksichtigung der Ephemeriden. 4 Taf. Greifswald, 1883.

=Oudemans, J. T.= Beiträge zur Kenntniss der Thysanura und Collembola.
(Bijdragen tot de Dierkunde, pp. 149–226. Amsterdam, 1888, 3 Taf.)

=Smith, John B.= An essay on the development of the mouth-parts of
certain insects. (Trans. Amer. Philosophical Soc., xix, pp. 175–198,
3 Pls.)

Also articles by Chatin, McLachlan, Riley, Wood-Mason.

_b._ Thysanoptera (Physapoda)

=Jordan, Karl.= Anatomie und biologie der Physapoda. (Zeitschr. f.
wissens. Zool., xlvii, pp. 541–620, 3 Taf. 1888.)

=Garman, H.= The mouth-parts of the Thysanoptera. (Bull. Essex Inst.,
xxii, 4 pp., Fig. 1890.)

—— The asymmetry of the mouth-parts of Thysanoptera. (Amer. Naturalist,
July, 1896, pp. 591–593, Fig.)

=Bohls, J.= Die Mundwerkzeuge der Physapoden. Dissertation Göttingen,
1891, pp. 1–36.

=Uzel, Heinrich.= Monographie der Ordnung Thysanoptera. Königgrätz,
1895, pp. 472, 10 Taf., 9 Figs.

_c._ Hemiptera

=Léon, N.= Beiträge zur Kenntniss der Mundteile der Hemipteren. Jena,
1887, pp. 47, 1 Taf.

—— Labialtaster bei Hemipteren. (Zool. Anzeiger, pp. 145–147, 1892, 1
Fig.)

—— Beiträge zur Kenntniss des Labiums der Hydrocoren. (Zool. Anzeiger,
März 29, 1897, pp. 73–77, Figs. 1–5.)

=Geise, O.= Mundteile der Rhynchoten. (Archiv f. Naturgesch., xlix,
1883, pp. 315–373, 1 Taf.)

=Wedde, Hermann.= Beiträge zur Kenntniss des Rhynchotenrüssels. (Archiv
f. Naturgesch., li Jahrg., 1 Bd., 1885, pp. 113–148, 2 Taf.)

=Smith, John B.= The structure of the hemipterous mouth. (Science, April
1, 1892, pp. 189–190, Figs. 1–5.)

_d._ Coleoptera

=Smith, John B.= The mouth-parts of _Copris carolina_; with notes on the
homologies of the mandibles. (Trans. Amer. Ent. Soc., xix, April,
1892, pp. 83–87, 2 Pls.)

_e._ Lepidoptera

=Kirbach, P.= Ueber die Mundwerkzeuge der Schmetterlinge. (Zool.
Anzeiger, vi Jahrg., 1883, pp. 553–558, 2 Figs.)

—— Ueber die Mundwerkzeuge der Schmetterlinge. (Archiv f.
Naturgeschichte, 1884, pp. 78–119, 2 Taf.)

=Walter, Alfred.= Palpus maxillaris Lepidopterorum. (Jenaische Zeitschr.
f. Naturwiss, xviii, 1884, pp. 121–173, Taf.)

—— Beiträge zur Morphologie der Lepidoptera. I, Mundteile. (Jenaische
Zeitschr. f. Naturwiss, xviii, 1885, pp. 751–807, 2 Taf.)

=Breitenbach, W.= Vorläufige Mitteilung über einige neue Untersuchungen
an Schmetterlingsrüsseln. (Archiv f. mikroskop. Anatomie, xiv, 1877,
pp. 308–317, 1 Taf.)

—— Untersuchungen an Schmetterlingsrüsseln. (Ibid., xv, 1878, pp. 8–29,
1 Taf.)

—— Ueber Schmetterlingsrüssel. (Entomolog. Nachr. 5 Jahrg., 1879, pp.
237–243, 1 Taf.)

—— Der Schmetterlingsrüssel. (Jenaische Zeitschr. f. Naturwiss, 1881.)

_f._ Siphonaptera

=Kräpelin, K.= Ueber die systematische Stellung der Puliciden.
(Festschrift z. 50 jahr. Jubil. d. Realgymnas. Iohanneum, Hamburg,
pp. 17, 1 Taf. 1884.)

=Kellogg, V. L.= The mouth-parts of the Lepidoptera. (Amer. Nat., xxix,
1895, pp. 546–556, 1 Pl. and Fig.)

_g._ Diptera

=Menzbier, Michael Alexander.= Ueber das Kopfskelett und die
Mundwerkzeuge der Zweiflügler. (Bull. Soc. Imp. Natur. de Moscou,
lv, 1880, pp. 8–71, 2 Taf.)

=Dimmock, George.= The anatomy of the mouth-parts and of the sucking
apparatus of some Diptera. Boston, 1881, pp. 48, 4 Pls.

=Meinert, F.= Fluernes Munddele. Trophi Dipterorum. Kjöbenhavn, 1881, 6
Pls.

—— Die Mundteile der Dipteren. (Zool. Anz. 1882, pp. 570–574, 599–603.)

=Becher, E.= Zur Kenntniss der Mundteile der Dipteren. (Denkschr. Akad.
d. Wissensch. Wien., xlv, 1882, pp. 123–162, 4 Taf.)

=Hansen, H. J.= Fabrica oris dipterorum: Dipterernes mund: anatomisk og
systematisk henseende. 1 Tabanidae, Bombyliidae, Asilidae, Thereva,
Mydas, Apiocera. (Naturhist. Tidsskrift, 1883, xiv, pp. 1–186, Taf.
1–5.)

=Kräpelin, Karl.= Zur Anatomie und Physiologie des Rüssels von Musca.
(Zeitschr. f. wissensch. Zool., xxxix, 1883, pp. 683–719, 2 Taf.)

=McCloskie, George.= Kraepelin’s Proboscis of the house-fly. (American
Naturalist, xviii, 1884, pp. 1234–1244, Figs.)

=Langhoffer, August.= Beiträge zur Kenntniss der Mundtheile der
Dipteren. Jena, 1888, pp. 1–32.

=Smith, John B.= A contribution toward a knowledge of the mouth-parts of
the Diptera. (Trans. Amer. Ent. Soc., xvii, Nov. 1890, pp. 319–339,
Figs. 1–22.)

_h._ Hymenoptera

=Briant, Travers J.= On the anatomy and functions of the tongue of the
honey-bee (worker). (Journ. Linn. Soc., London, xvii, 1884, pp.
408–416, 2 Pls.)

=Breithaupt, P. F.= Ueber die Anatomie und die Funktionen der
Bienenzunge. (Archiv f. Naturgesch., Jahrg. lii, 1886, pp. 47–112, 2
Taf.)

_i._ Larval stages

=Brauer, F.= Die Zweiflügler des kaiserlichen Museums zu Wien. III,
Systematische Studien auf Grundlage der Dipterenlarven nebst einer
Zusammenstellung von Beispielen aus der Litteratur uber dieselben
und Beschreibung neuer Formen. (Denkschr. math.-naturwiss. Cl. k.
Akad. Wiss. Wien, 1883, xlvii, pp. 100, 5 Taf.)

=Dewitz, H.= Ueber die Führung an den Körperanhangen der Insekten
speziell betrachtet an der Legescheide der Acridier, dem Stachel der
Meliponem und den Mundteilen der Larve von Myrmeleon, nebst
Beschreibung dieser Organe. (Berliner ent. Zeitschr., xxvi, 1882,
pp. 51–68.)

—— Die Mundteile der Larve von Myrmeleon. (Sitzungsber. d. Ges.
naturforsch. Freunde zu Berlin, 1881, pp. 163–166.)

=Redtenbacher, Josef.= Uebersicht der Myrmeleonidenlarven. (Denkschrift,
math.-naturwiss. Cl. k. Akad. Wiss. Wien, 1884, xlviii, pp. 335–368,
7 Taf.)

=Schiödte, J. G.= De metamorphosi Eleutheratorum. Bidrag til insekternes
udviklingshistorie. (Kroyer’s Naturhist. Tidsskrift. Kjöbenhavn. 12
Teile mit 88 Taf., 1862–1883.)

_j._ Embryonic stages

=Heymons, Richard.= Grundzüge der Entwicklung und des Körpersbaues von
Odonaten und Ephemeridem (Anhang zu den Abhandl. K. Akad. d.
Wissens. Berlin, 1896, p. 22, 2 Taf. See Figs. 5, 29.)

—— Entwicklungsgeschichtliche Untersuchungen an _Lepisma saccharina_ L.
(Zeitschr. f. Wissens. Zoologie, lxii, 1897, p. 595, 2 Taf. See Fig.
10.)

THE THORAX AND ITS APPENDAGES

_a._ The thorax; its external anatomy

The middle region of the body is called the thorax, and in general consists of three segments, which are respectively named the _prothorax_, _mesothorax_, and _metathorax_ (Figs. 88, 89, 98).

FIG. 88.—External anatomy of _Melanoplus spretus_, the head and thorax
disjointed.
]

The thorax contains the muscles of flight and those of the legs, besides the fore intestine (œsophagus and proventriculus), as well as, in the winged insects, the salivary glands.

In the more generalized orders, notably the Orthoptera, the three segments are distinct and readily identified.

FIG. 89.—Locust, Melanoplus, side view, with the thorax separated from
the head and abdomen, and divided into its three segments.
]

Each segment consists of the _tergum_, _pleurum_, and _sternum_. In the prothorax these pieces are not subdivided, except the pleural; in such case the tergum is called the _pronotum_. The prothorax is very large in the Orthoptera and other generalized forms, as also in the Coleoptera, but small and reduced in the Diptera and Hymenoptera. In the winged forms the tergum of the mesothorax is differentiated into four pieces or plates (sclerites). These pieces were named by Audouin, passing from before backwards, the _præscutum_, _scutum_, _scutellum_, and _postscutellum_. In the nymph stage and in the wingless adults of insects such as the Mallophaga, the true lice, the wingless Diptera, ants, etc., these parts by disuse and loss of the wings are not differentiated. It is therefore apparent that their development depends on that of the muscles of flight, of which they form the base of attachment. The scutum is invariably present, as is the scutellum. The former in nearly all insects constitutes the larger part of the tergum, while the latter is, as its name implies, the small shield-shaped piece directly behind the scutum.

FIG. 90.—Thorax of _Telea polyphemus_, side view, pronotum not
represented: _em_, epimerum of prothorax, the narrow piece above
being the prothoracic episternum; _ms_, mesoscutum; _scm_,
mesoscutellum; _ms″_, metascutum; _scm‴_, metascutellum; _pt_, a
supplementary piece near the insertion of tegulæ; _w_, pieces
situated at the insertion of the wings, and surrounded by membrane;
_epm″_, episternum of the mesothorax; _em″_, epimerum of the same;
_epm‴_, episternum of the metathorax; _em‴_ epimerum of the same,
divided into two pieces; _c′_, _c″_, _c‴_, coxæ; _te′_, _te″_,
_te‴_, trochantines; _tr_, _tr_, _tr_, trochanters. _A_, tergal view
of the mesothorax of the same; _prm_, præscutum; _ms_, scutum;
_scm_, scutellum; _ptm_, postscutellum; _t_, tegula.
]

The præscutum and postscutellum are usually minute and crowded down out of sight between the opposing segments. As seen in Fig. 90, the præscutum of most moths (Telea) is a small rounded piece, bent vertically down so as not to be seen from above. In Polystœchotes and also in Hepialus the præscutum is large, well-developed, triangular, and wedged in between the two halves of the scutum. The postscutellum is still smaller, usually forming a transverse ridge, and is rarely used in taxonomy.

FIG. 91.—Thorax of the house-fly: _prn_, pronotum; _prsc_, præscutum;
_sc′_, mesoscutum; _sct′_, mesoscutellum; _psct′_, postscutellum;
_al_, insertion of squama, extending to the insertion of the wings,
which have been removed; _msphr_, mesophragma; _h_, balancer
(halter); _pt_, tegula; _mtn_, metanotum; _epis_, _epis′_, _epis″_,
episternum of pro-, meso-, and metathorax; _epm′_, _epm″_, meso- and
meta-epimerum; _st′_, _st″_, meso- and metasternum; _cx′_, _cx″_,
_cx‴_, coxæ; _tr′_, _tr″_, _tr‴_, trochanters of the three pairs of
legs; _sp′_, _sp″_, _sp‴_, _sp‴′_, _sp‴″_, first to fifth spiracles;
_tg′_, _tg″_, tergites of first and second abdominal segments; _u′_,
_u″_, urites.
]

The metathorax is usually smaller and shorter than the mesothorax, being proportioned to the size of the wings. In certain Neuroptera and in Hepialidæ and some tineoid moths, where the hind wings are nearly as large as those of the anterior pair, the metathorax is more than half or nearly two-thirds as large as the mesothorax. In Hepialidæ the præscutum is large and distinct, while the scutum is divided into two widely separated pieces. The postscutellum is nearly or quite obsolete.

The pleurum in each of the three thoracic segments is divided into two pieces; the one in front is called the _episternum_, since it rests upon the sternum; the other is the _epimerum_. To these pieces, with the sternum in part, the legs are articulated (Fig. 89).

Between the episterna is situated the breastplate or _sternum_, which is very large in the more primitive forms, as the Orthoptera, and is small in the Diptera and Hymenoptera.

FIG. 92.—Prothorax of _Geometra papilionaria_: _n_, notum; _p_,
pleura; _st_, sternum; _pt_, patagia; _m_, membrane; _f_, femur;
_h_, a hook bent backwards and beneath, and connecting the pro- with
the mesothorax.—After Cholodkowsky.
]

The episterna and epimera are in certain groups, Neuroptera, etc., further subdivided each into two pieces (Fig. 102). The smaller pieces, hinging upon each other and forming the attachments of the muscles of flight, differ much in shape and size in insects of different orders. The difference in shape and degree of differentiation of these parts of the thorax is mentioned and illustrated under each order, and reference to the figures will obviate pages of tedious description. A glance, however, at the thorax of a moth, fly, or bee, where these numerous pieces are agglutinated into a globular mass, will show that the spherical shape of the thorax in these insects is due to the enlargement of one part at the expense of another; the prothoracic and metathoracic segments being more or less atrophied, while the mesothorax is greatly enlarged to support the powerful muscles of flight, the fore wings being much larger than those appended to the metathorax. In the Diptera, whose hinder pair of wings are reduced to the condition of halteres, the reduction of the metathorax as well as prothorax is especially marked (Fig. 91).

=The patagia.=—On each side of the pronotum of Lepidoptera are two transversely oval, movable, concavo-convex, erectile plates, called _patagia_ (Fig. 92). On cutting those of a dry Catocala in two, they will be seen to be hollow. Cholodkowsky[19] states that they are filled with blood and tracheal branches; and he went so far as to regard them as rudimentary prothoracic wings, in which view he was corrected by Haase,[20] who compares them with the tegulæ, regarding them also as secondary or accessory structures.

=The tegulæ.=—On the mesothorax are the _tegulæ_ of Kirby (_pterygodes_ of Latreille, _paraptera_ of McLeay, _hypoptère_ or _squamule_), which cover the base of the fore wings, and are especially developed in the Lepidoptera (Fig. 90, _A_, _t_) and in certain Hymenoptera (Fig. 95, _c_).

The external opening of the spiracles just under the fore wings, is situated in a little plate called by Audouin the _peritreme_.

FIG. 93.—Transformation of the bumble bee, Bombus, showing the
transfer of the 1st abdominal larval segment (_c_) to the thorax,
forming the propodeum of the pupa (_D_) and imago; _n_, spiracle of
the propodeum. _A_, larva; _a_, head; _b_, 1st thoracic; _c_, 1st
abdominal segment. _B_, semipupa; _g_, antenna; _h_, maxillæ; _i_,
1st; _j_, 2d leg; _k_, mesoscutum; _l_, mesoscutellum; _m_,
metathorax; _d_, urite (sternite of abdomen); _e_, pleurite; _f_,
tergite; _o_, ovipositor; _r_, lingua; _q_, maxilla.
]

In the higher or aculeate Hymenoptera, besides the three segments normally composing the thorax, the basal abdominal segment is during the change from the larva to the pupa transferred to this region, making four segments. This first abdominal is called “the median segment” (Figs. 93–95). In such a case the term _alitrunk_ has been applied to this region, _i.e._ the thorax, as thus constituted. Latreille wrongly stated that in the Diptera the first abdominal segment also entered into the composition of the thorax; but Brauer has fully disproved that view, as may be seen by an examination of his sketches which we have copied (Fig. 94).

FIG. 94.—7, 8, thorax of _Tipula gigantea_; 9, of Leptis; 10, thorax
of _Tabanus bromius_ after the removal of the abdomen, in order to
bring into view the inner mesophragma (_f_), and to show the
extension of the metathorax _g_ and _g′_; _tr_, trochanter; 11, hind
end of the mesothorax, the entire metathorax, and the 1st and 2d
abdominal segments of _Volucella zonaria_, seen from the side. The
internal mesophragma (_f_), and the position of the muscle inserted
in it, are indicated by the two lines _M_. _p_, Callus postalaris;
_pr_ (_pz_ in 8), callus præalaris Osten Sacken (= “patagium” of
some authors); _g_, metanotum; _g′_, metepimerum, “segment médiaire”
of Latreille (wrongly considered by him to be the 1st abdominal
segment); 4, metasternum (hypopleura of Osten Sacken); 5 (?
“episternum of metathorax” (Brauer) = metapleura of Osten Sacken);
6, and also _H_, halter; _st_^1, mesothoracic stigma; _st_^2,
metathoracic stigma; _st_^3, first abdominal stigma; γ,
dorsopleural; δ, sternopleural; ε, mesopleural sutures; _h_, 1st,
_i_, 2d, abdominal segment; _al_, wing; _alul_, alula. 12, the head
and the three thoracic rings, and the 1st abdominal segment of
_Ephemera vulgata_, the connecting membranes are in white: _a_,
prothorax; _b_, præscutum; _c_, scutum; _d_, scutellum; _e_,
postscutellum; _ps_, postscutellum of mesothorax.—After Brauer.
]

FIG. 95.—Alitrunk of _Sphex chrysis_: _A_, dorsal aspect; _a_,
pronotum; _b_, mesonotum; _c_, tegula; _d_, base of fore,—_e_, of
hind, wing; _f_, _g_, divisions of metanotum; _h_, median (true
first abdominal) segment; _i_, its spiracle; _k_, second abdominal
segment, usually called the petiole or first abdominal segment. _B_,
posterior aspect of the median segment; _a_, upper part; _b_,
superior,—_c_, inferior, abdominal foramen; _d_, ventral plate of
median segment; _e_, coxa.—After Sharp.
]

The sternum is in rare cases subdivided into two halves, as in the meso- and metathorax of the cockroach; in Forficula the prosternum is divided into four pieces besides the sternum proper (Fig. 96); and in Embia, also, the sternites, according to Sharp, are complex.

FIG. 96.—Sternal view of pro-, meso-, and metathorax of _Forficula
tæniata_: _pst_, præsternum, divided into 4 pieces; _st_, pro-,
_st′_, meso-, _st″_, metasternum; _cx_, coxa; _not_, notum.
]

FIG. 97.—_A_, under surface of prothorax, or prosternum, of _Dyticus
circumflexis_: 2._g_, prosternum; 2._f_, episternum; 2._h_,
epimerum; 2._s_, antefurca or entothorax.
]

FIG. 98.—Meso- (_G__{2}) and metathoracic ganglia (_G__{1}), with the
apodemes of Gryllotalpa.—After Graber.
]

FIG. 99.—Parts of the mesothorax of Dyticus: _A_, mesosternum; 3._a_,
præscutum; 3._b_, scutum; 3._c_, scutellum; 3._d_, postscutellum;
3._e_, parapteron; 3._g_, mesosternum; 3._f_, episternum; 3._h_,
epimerum; 3._s_, medifurca or entothorax.
]

FIG. 100.—Parts of the metathorax of Dyticus: _A_, metasternum; 4._a_,
præscutum; 4._b_, scutum; 4._c_, scutellum; 4._d_, postscutellum;
4._e_, parapteron; 4._f_, episternum; 4._g_, metasternum; 4._h_,
epimerum; 4._s_, postfurca.—This and Figs. 97 and 99 from Audouin,
after Newport.
]

=The apodemes.=—The thorax is supported within by beam-like processes, or _apodemes_, which pass inward and also form attachments for the muscles. Those passing up from the sternum form the _entothorax_ of Audouin, and the process of each thoracic segment is called respectively the _antefurca_, _medifurca_, and _postfurca_. In the Orthoptera (Caloptenus and Anabrus), the antefurca is large, thin, flattened, directed forward, and bounds each side of the prothoracic ganglion. In the Coleoptera two plates (Fig. 97, 2._s_) arise from the inside of the sternum and “form a collar or leave a circular hole between them for the passage of the nervous cord” (Newport). The medifurca is a pair of flat processes which diverge and bridge the commissure, while the postfurca is situated under the commissure. In beetles (Dyticus) Newport states that it is expanded into two broad plates, to which the muscles of the posterior legs are attached. Graber also notices in the mole cricket between the apodemes of the meso- and metathorax, a flattened spine (Fig. 98, _do_) with two perforations through which pass the commissures connecting the ganglia. Besides these processes there are large, thin, longitudinal partitions passing down from the tergum (or dorsum), called _phragmas_; they are most developed in those insects which fly best, _i.e._ in Coleoptera (Figs. 97–101), Lepidoptera, Diptera, and Hymenoptera, none being developed in the prothorax. (The term _phragma_ has also been applied to a partition formed by the inflexed hinder edge of this segment, and is present only in those insects in which the prothorax is movable.—Century Dictionary.) All these ingrowths may be in general termed _apodemes_. There are similar structures in Crustacea and also in Limulus; but Sharp restricts this term to minute projections in beetles (Goliathus) situated at the sides of the thorax near the wings. (Insecta, p. 103, Fig. 57.) The internal processes arising from the sternal region have been called _endosternites_.

FIG. 101.—Internal skeleton of _Lucanus cervus_, ♂, head: _A_,
antenna; _f_, mandible; _d_, mentum; 2, 4, tendons of mandible; _f_,
_u_, _t_, parts of the tentorium; 3 _e_, labial muscles. Thorax: 2,
prothorax; 3, 4, meso- and metathorax fused solidly together; 3 _r_,
acetabulum of prothorax, into which the coxa is inserted; 2 _s_,
sternum; 3_t_, acetabulum of mesothorax, 4_r_, of metathorax; 3 _s_,
mesothoracic sternum fused with that of the metathorax (4_g_); 4
_s_, apodeme.—After Newport.
]

=The acetabula.=—These are the cavities in which the legs are inserted. They are situated on each side of the posterior part of the sternum, in each of the thoracic segments. They are, in general, formed by an approximation of the sternum and epimerum, and sometimes, also, of the episternum, as in Dyticus (Fig. 97, _A_). This consolidation of parts, says Newport, gives an amazing increase of strength to the segments, and is one of the circumstances which enables the insect to exert an astonishing degree of muscular power.

TABULAR VIEW OF THE SEGMENTS, PIECES, AND APPENDAGES OF THE THORAX

═══════════════════════╤═══════════════════════╤═══════════════════════
NAME OF SEGMENT │ PIECES (SCLERITES) │ APPENDAGES
───────────────────────┼───────────────────────┼───────────────────────
1. Prothorax │Pronotum, sometimes │
│ differentiated into │
│Scutum │1st pair of legs
│Scutellum │Patagia
│Episternum │
│Epimerum │
│Sternum │
│Antefurca │
│ │
2. Mesothorax │Præscutum │
│Scutum │2d pair of legs
│Scutellum │1st pair of wings
│Proscutellum │Tegulæ
│Episternum │Squamæ (Alulæ)
│Epimerum │Peritreme
│Sternum │
│Mesofurca │
│Mesophragma │
│Apodemes │
│ │
3. Metathorax │Præscutum │
│Scutum │3d pair of legs
│Scutellum │2d pair of wings
│Postscutellum │(Halteres of Diptera)
│Episternum │
│Epimerum │
│Sternum │
│Postfurca │
│Metaphragma │
│Apodemes │
───────────────────────┴───────────────────────┴───────────────────────

FIG. 102.—External anatomy of the trunk of _Hydröus piceus_: _A_,
sternal—_B_, tergal aspect; 2, pronotum; 2 _a_, prosternum; 2 _f_,
episternum; 3 _a_, præscutum; 3 _b_, scutum; 3 _c_, scutellum; 3
_d_, postscutellum; 3 _g_, mesosternum; 3 _h_, episternum; 3 _f_,
epimerum; 3 _i_, crest of the mesosternum; 3 _a_, parapteron; 3 _k_,
coxa; 4 _a_, metapræscutum; 4 _b_, metascutum; 4 _c_, metascutellum;
4 _d_, postscutellum; 4 _e_, tegula; 4 _f_, episternum; 4 _h_,
epimerum; 4 _g_, metasternum; 4 _i_, crest of metasternum; 4 _k_ and
_l_, coxa; 4 _m_, trochanter; _n_, femur; _o_, tibia; _p_, tarsus;
_q_, unguis; 7–11, abdominal segments.—After Newport.
]

_b._ The legs: their structure and functions

The mode of insertion of the legs to the thorax is seen in Figs. 90, 97, 101, and 103. They are articulated to the episternum, epimerum, and sternum, taken together, and consist of five segments. The basal segment or joint is the _coxa_, situated between the episternum and trochanter. The coxa usually has a posterior subdivision or projection, the _trochantine_; sometimes, as in Mantispa (Fig. 103), the trochantine is obsolete. We had previously supposed that the trochantine was a separate joint, but now doubt whether it represents a distinct segment of the leg, and regard it as only a subdivision of the coxa. It is attached to the epimerum, and is best developed in Panorpidæ, Trichoptera, and Lepidoptera. In the Thysanura the trochantine is wanting, and in the cockroach it merely forms a subdivision of the coxa, its use being to support the latter. The second segment is the trochanter, a more or less short spherical joint on which the leg proper turns; in the parasitic groups (Ichneumonidæ, etc., Fig. 104) it is usually divided into two pieces, though there are some exceptions. The trochanter is succeeded by the _femur_, _tibia_, and _tarsus_, the latter consisting of from one to five segments, the normal number being five. Tuffen West believed that the pulvillus is the homologue of an additional tarsal joint, “a sixth tarsal joint.” The last tarsal segment ends in a pair of freely movable claws (ungues), which are modified setæ; between the claws is a cushion-like pad or adhesive lobe, called the _empodium_ or _pulvillus_ (Fig. 105, also variously called _arolium_, _palmula_, _plantula_, _onychium_, its appendage being called _paronychium_ and also _pseudonychium_). It is cleft or bilobate in many flies, but in Sargus trilobate. All these parts vary greatly in shape and relative size in insects of different groups, especially Trichoptera, Lepidoptera, Diptera, and Hymenoptera. In certain flies (_e.g._ Leptogaster) the empodium is wanting (Kolbe). By some writers the middle lobe is called the empodium and the two others pulvilli.

FIG. 103.—Side view of meso- and metathorax of _Mantispa brunnea_,
showing the upper and lower divisions of the epimerum (_s. em′_, _s.
em″_, _i. em′_, _i. em″_); _s. epis_, _i. epis″_, the same of the
episternum.
]

FIG. 104.—Divided (ditrochous) trochanter of an ichneumon: _cx_, coxa;
_tr_, the two divisions of the trochanter; _f_, femur.—After Sharp.
]

The fore legs are usually directed forward to drag the body along, while the middle and hind legs are directed outward and backward to push the body onwards. While arachnids walk on the tip ends of their feet, myriopods, Thysanura, and all larval insects walk on the ends of the claws, but insects generally, especially the adults, are, so to speak, plantigrade, since they walk on all the tarsal joints. In the aquatic forms the middle and hind tarsi are more or less flattened, oar-like, and edged with setæ. In leaping insects, as the locusts and grasshoppers, and certain chrysomelids, the hind femora are greatly swollen owing to the development of the muscles within. The tibia, besides bearing large, lateral, external spines, occasionally bears at the end one or more spines or spurs called _calcaria_. The fore tibia also in ants, etc., bear tactile hairs, and chordotonal organs, as well as other isolated sense-organs (Janet), and, in grasshoppers, ears.

In the Carabidæ the legs are provided with combs for cleaning the antennæ (Fig. 107), and in the bees and ants these cleansing organs are more specialized, the pectinated spine (_calcar_) being opposed by a tarsal comb (Fig. 106, _d_; for the wax-pincers of bees, see _g_). In general the insects use their more or less spiny legs for cleansing the head, antennæ, palpi, wings, etc., and the adaptations for that end are the bristles or spinules on the legs, especially the tibiæ.

FIG. 105.—Foot of honey-bee, with the pulvillus in use: _A_, under
view of foot; _t_, _t_, 3d–5th tarsal joints; _a n_, unguis; _f h_,
tactile hairs; _p v_, pulvillus; _cr_, curved rod. _B_, side view of
foot. _C_, central part of sole; _pd_, pad; _cr_, curved rod; _pv_,
pulvillus unopened.—After Cheshire.
]

FIG. 106.—Modifications of the legs of different bees. _A_, Apis: _a_,
wax-pincer and outer view of hind leg; _b_, inner aspect of
wax-pincer and leg, with the nine pollen-brushes or rows of hairs;
_c_, compound hairs holding grains of pollen; _d_, anterior leg,
showing antenna-cleaner; _e_, spur on tibia of middle leg. _B_,
Melipona: _f_, peculiar group of spines at apex of tibia of hind
leg; _g_, inner aspect of wax-pincer and first tarsal joint. _C_,
Bombus: _h_, wax-pincer; _i_, inner view of the same and first
tarsal joint, all enlarged.—From _Insect Life_, U. S. Div. Ent.
]

Osten Sacken states that among Diptera the aerial forms (Bombylidæ, etc.) with their large eyes or holoptic heads, which carry with them the power of hovering or poising, have weak legs, principally fit for alighting. On the other hand, the pedestrian or walking Diptera (Asilidæ, etc.) “use the legs not for alighting only, but for running, and all kinds of other work, seizing their prey, carrying it, climbing, digging, etc.; their legs are provided not only with spines and bristles, but with still other appendages, which may be useful, or only ornamental, as secondary sexual characters.”

FIG. 107.—End of tibia and tarsal joints of Anophthalmus; _c_, comb.
]

=Tenent hairs.=—Projecting from the lower surface of the empodium are the numerous “tenent hairs,” or holding hairs, which are modified glandular setæ swollen at the end and which give out a minute quantity of a clear adhesive fluid (Figs. 108, 109, 130, 134). In larval insects, and the adults of certain beetles, Coccidæ, Aphidæ, and Collembola, which have no empodium, there are one or more of these tenent hairs present. They enable the insect to adhere to smooth surfaces.

FIG. 108.—Transverse section through a tarsal joint of Telephorus, a
beetle: _ch_, cuticula of the upper side; _m_, its matrix; _ch′_,
the sole; _m′_, its matrix; _h_, adhesive hair; _h′_, tactile hair,
supplied with a nerve (_n′_), and arising from a main nerve (_n_);
_n″_, ganglion of a tactile hair; _t_, section of main trachea, from
which arises a branch (_t′_); _dr_, glands which open into the
adhesive hairs, and form the sticky secretion; _e_, chitinous
thickening; _s_, sinew; _b_, membrane dividing the hollow space of
the tarsal joint into compartments. See p. 111.—After Dewitz.
]

Striking sexual secondary characters appear in the fore legs of the male Hydrophilus, the insect, as Tuffen West observes, walking on the end of the tibia alone and dragging the tarsus after it. The last tarsal joint is enlarged into the form of an irregular hollow shield. The most completely suctorial feet of insects are those of the anterior pair of Dyticus (Fig. 132). The under side of the three basal joints is fused together and enlarged into a single broad and nearly circular shield, which is convex above and fringed with fine branching hairs, and covered beneath with suckers, of which two are exceptionally large; by this apparatus of suckers the male is enabled to adhere to the back of its mate during copulation. The line branching hairs around the edge prevent the water from penetrating and thus destroying the vacuum, “while if the female struggle out of the water, by retaining the fluid for some time around the sucker, they will in like manner under these altered conditions equally tend to preserve the effectual contact.” (Tuffen West.)

FIG. 109.—Cross-section through tarsus of a locust: _ch_, cuticula of
upper side,—_ch′_, _ch″_, _ch‴_, of sole; _ch_, tubulated layer;
_ch″_, lamellate layer; _ch‴_, inner projections of _ch″_. Other
lettering as in Fig. 101. See p. 113.—After Dewitz.
]

In the saw-flies (Uroceridæ and Tenthredinidæ) and other insects, there are small membranous oval cushions (_arolia_, Figs. 109 and 131) beneath each or nearly each tarsal joint.

The triunguline larvæ of the Meloidæ are so called from apparently
having three ungues, but in reality there is only a single claw,
with a claw-like bristle on each side.

=Why do insects have but six legs?=—Embryology shows that the
ancestors of insects were polypodous, and the question arises to
what cause is due the process of elimination of legs in the
ancestors of existing insects, so that at present there are no
functional legs on the abdomen, these being invariably restricted
(except in caterpillars) to the thorax, and the number never being
more than six. It is evident that the number of six legs was fixed
by heredity in the Thysanura, before the appearance of winged
insects. We had thought that this restriction of legs to the thorax
was in part due to the fact that this is the centre of gravity, and
also because abdominal legs are not necessary in locomotion, since
the fore legs are used in dragging the insect forwards, while the
two hinder pairs support and push the body on. Synchronously with
this elimination by disuse of the abdominal legs, the body became
shortened, and subdivided into three regions. On the other hand, as
in caterpillars, with their long bodies, the abdominal legs of the
embryo persist; or if it be granted that the prop-legs are secondary
structures, then they were developed in larval life to prop up and
move the abdominal region.

The constancy of the number of six legs is explained by Dahl as
being in relation to their function as climbing organs. One leg, he
says, will almost always be perpendicular to the plane when the
animal is moving up a vertical surface; and, on the other hand, we
know that three is the smallest number with which stable equilibrium
is possible; an insect must therefore have twice this number, and
the great numerical superiority of the class may be associated with
this mechanical advantage. (This numerical superiority of insects,
however, seems to us to be rather due to the acquisition of wings,
as we have already stated on pages 2 and 120.)

=Loss of limbs by disuse.=—Not only are one or both claws of a single pair, or those of all the feet atrophied by disuse, but this process of reduction may extend to the entire limb.

In a few insects one of the claws of each foot is atrophied, as in
the feet of the Pediculidæ, of many Mallophaga, all of the Coccidæ,
in Bittacus, Hybusa (Orthoptera), several beetles of the family
Pselaphidæ, and a weevil (Brachybamus). Hoplia, etc., bear but a
single claw on the hind feet, while the allied Gymnoloma has only a
single claw on all the feet. Cybister has in general a single
immovable claw on the hind feet, but _Cybister scutellaris_ has,
according to Sharp, on the same feet an outer small and movable
claw. In the water bugs, Belostoma, etc., the fore feet end in a
single claw, while in others (Corisa) both claws are wanting on the
fore feet. Corisa also has no claws on the hind feet; Notonecta has
two claws on the anterior four feet, but none on the hind pair. In
Diplonychus, however, there are two small claws present. (Kolbe.)

FIG. 110.—Last tarsal joint of _Melolontha vulgaris_, drawn as if
transparent to show the inner mechanism: _un_, claws; _str_,
extensor plate; _s_, tendon of the flexor muscle; _vb_, elastic
membrane between the extensor plate and the sliding surface _u_;
_krh_, process of the ungual joint; _emp_, extensor spine, and _th_,
its two tactile hairs.—After Ockler, from Kolbe.
]

Among the Scarabæidæ, the individuals of both sexes of the fossorial genus Ateuchus (_A. sacer_) and eight other genera, among them _Deltochilum gibbosum_ of the United States, have no tarsi on the anterior feet in either sex. The American genera Phanæus (Fig. 111), Gromphas, and Streblopus have no tarsal joints in the male, but they are present in the female, though much reduced in size, and also wanting, Kolbe states, in many species of Phanæus. The peculiar genus Stenosternus not only lacks the anterior feet, but also those of the second and third pair of legs are each reduced to a vestige in the shape of a simple, spur-like, clawless joint. The ungual joint is wanting in the weevil Anoplus, and becomes small and not easily seen in four other genera.

Ryder states that the evidence that the absence of fore tarsi in
Ateuchus is due to the inheritance of their loss by mutilation is
uncertain. Dr. Horn suggests that cases like Ateuchus and
Deltochilum, etc., “might be used as an evidence of the persistence
of a character gradually acquired through repeated mutilation, that
is, a loss of the tarsus by the digging which these insects
perform.” On the other hand, the numerous species of Phanæus do
quite as much digging, and the anterior tarsi of the male only are
wanting. “It is true,” he adds, “that many females are seen which
have lost their anterior tarsi by digging; have, in fact, worn them
off; but in recently developed specimens the front tarsi are always
absent in the males and present in the females. If repeated
mutilation has resulted in the entire disappearance of the tarsi in
one fossorial insect, it is reasonable to infer that the same
results should follow in a related insect in both sexes, if at all,
and not in the male only. It is evident that some other cause than
inherited mutilation must be sought for to explain the loss of the
tarsi in these insects.” (Proc. Amer. Phil. Soc., Philadelphia,
1889, pp. 529, 542.)

FIG. 111.—Fore tibia of _Phanæus carnifex_, ♂, showing no trace of the
tarsus.
]

FIG. 112.—Fore leg of the mole-cricket: _A_, outer, _B_, inner,
aspect; _e_, ear-slit.—After Sharp.
]

The loss of tarsi may be due to disuse rather than to the inheritance of mutilations. Judging by the enlarged fore tibiæ, which seem admirably adapted for digging, it would appear as if tarsi, even more or less reduced, would be in the way, and thus would be useless to the beetles in digging. Careful observations on the habits of these beetles might throw light on this point. It may be added that the fore tarsi in the more fossorial Carabidæ, such as Clivina and Scarites, as well as those of the larva of Cicada and those of the mole crickets (Fig. 112), are more or less reduced; there is a hypertrophy of the tibiæ and their spines. The shape of the tibia in these insects, which are flattened with several broad triangular spines, bears a strong resemblance to the nails or claws of the fossorial limbs of those mammals which dig in hard soil, such as the armadillo, manis, aardvark, and Echidna. The principle of modification by disuse is well illustrated in the following cases.

In many butterflies the fore legs are small and shortened, and of little use, and held pressed against the breast. In the Lycænidæ the fore tarsi are without claws; in Erycinidæ and Libytheidæ the fore legs of the males are shortened, but completely developed in the females, while in the Nymphalidæ the fore legs in both sexes are shortened, consisting in the males of one or two joints, the claws being absent in the females. Among moths loss of the fore tarsi is less frequent. J. B. Smith[21] notices the lack of the fore tarsi in the male of a deltoid, _Litognatha nubilifasciata_ (Fig. 113), while the hind feet of _Hepialus hectus_ are shortened. In an aphid (_Mastopoda pteridis_, Esl.) all the tarsi are reduced to a single vestigial joint (Fig. 114).

FIG. 113.—Leg of Litognatha: _cx_, coxa; _f_, femur; _t_, tibia; _ep_,
its epiphysis, and _sh_, its shield-like process. The tarsus
entirely wanting.—After Smith.
]

Entirely legless adult insects are rare, and the loss is clearly seen to be an adaptation due to disuse; such are the females of the Psychidæ, the females of several genera of Coccidæ (Mytilaspis, etc.), and the females of the Stylopidæ.

Apodous larval insects are common, and the loss of legs is plainly seen to be a secondary adaptive feature, since there are annectant forms with one or two pairs of thoracic legs. All dipterous and siphonapterous larvæ, those of all the Hymenoptera except the saw-flies, a few lepidopterous larvæ, some coleopterous, as those of the Rhyncophora, Buprestidæ, Eucnemidæ, and other families, and many Cerambycidæ are without any legs. In _Eupsalis minuta_, belonging to the Brenthidæ, the thoracic legs are minute.

The legs of larvæ end in a single claw, upon the tips of which the insect stands in walking.

_c._ Locomotion (walking, climbing, and swimming)

=Mechanics of walking.=—To Graber we owe the best exposition of the mechanics of walking in insects.

“The first segment of the insect leg,” he says, “upon which the
weight of the body rests first of all, is the coxa. Its method of
articulation is very different from that of the other joints. The
enarthrosis affords the most extensive play, particularly in the
Hymenoptera and Diptera.”

In the former the development of their social conditions is very
closely connected with the freest possible use of the legs, which
serve as hands. In the beetles, however, which are very compactly
built, there exists a solid articulation whereby the entire hip
rests in a tent-like excavation of the thorax, and can only be
turned round a single axis, as may be seen in Fig. 115, where _c_
represents the imaginary revolving axis and _d_ the coxa. In the
case we are supposing, therefore, only a backward and forward
movement of the coxa is possible, the extent of the play of which
depends on the size of the coxal pan, as well as certain groin or
bar-like structures which limit further rotation. In the very
dissimilar arrangement which draws in the fore, middle, and hind
legs toward the body it is self-evident that their extent of action
is also different. This arrangement seems to be most yielding on the
fore legs, where the hips, to confine ourselves to the stag-beetles,
can be turned backward and forward 60° from the middle or normal
position, and therefore describe on the whole a curve of 120°. The
angle of turning on the middle leg hardly exceeds a legitimate
limit, yet a forward as well as a backward rotation takes place. The
former is entirely wanting in the hind hips; they can only be moved
backward.

FIG. 114.—Leg of an Aphid, with the tarsus (_t_) much reduced: 1, 2,
3, legs of 1st, 2d, and 3d pairs.
]

The number and strength of the muscles on which the rotation of the
hips depends, correspond with these varying movements of the
individual legs. Thus, according to Straus Durckheim, the fore coxa
of many beetles possesses five separate muscles and four forward and
one backward roll; the middle coxa a like number of muscles but only
two forward rolls, while the hind hips succeed in accomplishing each
of the motions named with a single muscle.

One can best see how these muscles undertake their work, and above
all how they are situated, if he lays bare the prothorax of the stag
beetle (Fig. 116). Here may be seen first the thick muscle which
turns to the front the rotating axis in its cylindrical pan, and
thus helps to extend the leg, while two other tendons, which take
the opposite direction, are fitted for reflex movements.

FIG. 115.—Mechanics of an insect’s leg: _d_, coxa,—_c_, axis of
revolution; _a_ and _b_, the coxal muscles; _e_, trochanter muscle
(elevator of the femur); _f_, extensor,—_g_, flexor, of the tibia
(_pn_); _n_, tibial spine; _h_, flexor.—_i_, extensor, of the
foot; _k_, extensor,—_l_, flexor, of the claw; _po_, place of
flexure of the tibia; _p^1q_, leg after being turned back by the
coxa.—_p^1r_, by the simultaneous flexure of the tibia. The
resulting motion of the end of the tibia, through the simultaneous
movement (_no_) and revolution (_nq_), indicates the curve
_nr_.—After Graber.
]

In Fig. 115 the muscles mentioned above, and their modes of working,
may be distinguished by the arrows _a_ and _b_.

In order to simplify matters, we will imagine the second component
part of the normal insect leg, _i.e._ the trochanter (Figs. 116,
117, _r_), as grown together with the third lever, _i.e._ the femur,
as the movement of both parts mostly takes place uniformly.

FIG. 116.—Section of the fore leg of a stag-beetle, showing the
muscles: _S_, extensor,—_B_, flexor, of the leg; _s_,
extensor,—_b_, flexor, of the femur; _o_, femur; _u_, tibia; _f_,
tarsus; _k_, claw; 109, _s_, extensor,—_b_, flexor, of the
femoro-tibial joint, both enlarged.—After Graber.
]

The pulling of the small trochanter muscle works against the weight
of the body when this is carried over on to the trochanter by means
of the coxa, as seen at the arrow _e_ in Fig. 115. It may be
designated as the femoral lever.

The plane of direction in which the femur, as seen by the rotation
just mentioned, is moved, exactly coincides in insects with that of
the tibia and the foot, while all can be simultaneously raised or
dropped, or, as the case may be, stretched out or retracted.
Therein, therefore, lies an essential difference from the fully
developed extremities of vertebrates among which, even on the lever
arms which are stationary at the end, an extensive turning is
possible.

The muscles which move the tibia, and indirectly the femur, also
consist of an extensor muscle which is situated in the upper side of
the femur (Fig. 116, _s_, Fig. 115, _f_), and of a flexor (Fig. 116,
_b_, Fig. 115, _g_), which lies under the former.

The stilt-like spines on the point (Figs. 115 and 118, _L_{3}n_) on
which this segment is directly supported are important parts of the
tibia. (Graber.)

FIG. 117.—Left fore leg of a cerambycid beetle: _h_, coxa; _r_,
trochanter; _o_, femur; _u_, tibia; _f_, tarsus; _k_, claw.—After
Graber.
]

Considering the respective positions of the individual levers of the leg and the nature of the materials of which they are made, the legs of insects may be likened, as Graber states, to elastic bows, which, when pressed down together from above, their own indwelling elasticity is able to raise again and thus keep the body upright.

This is very plainly shown in certain stilt-legged bark-beetles, in which, as in a rubber doll, as soon as the body is pressed down on the ground, the organs of motion extend again without the intervention of muscles; indeed this experiment succeeds even with dead, but not yet wholly stiff, insects.

Graber then turns to the analysis of the movements of insect legs when in motion, and the mode of walking of these insects in general. This subject had been but slightly investigated until Graber made a series of observations and experiments, of which we can give only the most important results.

The locomotion of insects is an extremely complicated subject.

Let us consider, Graber says, first, a running or carabid beetle,
when walking merely with the fore and hind legs. The former will be
bent forward and the latter backward.

“Let us begin with the left fore leg (Fig. 118, _L_{1}_). Let the
same be extended and fixed on the ground by means of its sharp claws
and its pointed heel. Now what happens when the tibial flexors draw
together? As the foot, and therefore the tibia also, have a firm
position, then the contraction of the muscles named must cause the
femur to approach the tibia, whereby the whole body is drawn along
with it. This individual act of motion may be well studied in
grasshoppers when they are climbing on a twig by stretching out
their long fore leg directly forward, and then drawing up the body
through the shortening of the tibial flexors until the middle leg
also reaches the branch.

“But while the fore legs advance the body by drawing the free lever
to the fixed leg-segment, the hind legs do this in exactly the
opposite way. The hind leg, namely, seeks to stretch out the tibia,
and thus to increase the angle of the knee (_R_{3})_, thereby giving
a push on the ground, by means of which the body is shoved forward a
bit.

“Though it might be supposed that the feet would remain stationary
during the extension or retraction of the limbs, this never occurs
in actual walking. Not merely the upper, but also the lower, thigh
is either drawn in or stretched out, as the case may be. The latter
then describes a straight line with its point during this scraping
or scratching motion (Fig. 115, _no_), which is obviously the chord
to that quadrant which would be drawn by the tibia or foot in a
yielding medium, as water, for instance. But even this motion
results extremely rarely, and never in actual walking. If we fix our
eye anew upon the fore leg at the very moment when it is again
retracted, after the resultant ‘fixing,’ we shall then observe that
the hip also is simultaneously turned backward in a definite angle.
The tibia would describe the arc _nq_ (Fig. 115) by means of the
latter alone.

“This plane, in conjunction with the rectilinear ‘movement’ (_no_)
obtained by the retraction of the tibia, produces a path (_nr_), and
this is what is actually described by a painted foot upon a properly
prepared surface, as a sheet of paper;[22] supposing, however, that
the body in the meantime is not moved forward by other forces. In
the last case, and this indeed always takes place in running, the
trunk is moved a bit forward, together with the leg which is just
describing its curve with a rapidity corresponding to the momentum
obtained; the result of this is that the curve of the foot from its
beginning (_n_) to its end (_a_) bends round close to itself, just
as a man who, when on board a ship in motion, walks across it
diagonally, and yet on the whole moves forward, because his line of
march, uniting with that of the ship, results in a change of
position in space.

“The case is the same in the middle and hind legs, which must make a
double course also, yet in such a way that the straight line is
drawn, not during the retraction, but during the extension; during
which, however, quite as in the fore leg, the members mentioned
(_R_{3}_) gradually approach the body.

“When the legs have reached the maximum of their retraction, or of
their extension, as the case may be, and therefore the end of their
active course for that time, then begins the opposite or backward
movement; that is, the fore legs are again extended, while their
levers draw the remaining legs together again.

FIG. 118.—A Carabus beetle in the act of walking or running: three
legs (_L_{1}_, _R_{2}_, _L_{3}_) are directed forward, while the
others (_R_{1}_, _L_{2}_, _R_{3}_), which are directed backward
toward the tail, have ended their activity; _ab_, _cd_, and _ef_
are curves described by the end of the tibiæ, and passing back to
the end of the body; _bh_, _di_, and _fg_ are curves described by
the same legs during their passive change of position.—After
Graber.
]

“At the same time, as we may see by the uniting leg, the limb is
either a little raised, that there may be no unnecessary friction,
or it remains during the passive step also, with its means of
locomotion in slight contact with the ground.

“The curve of two steps, as inscribed by the end of the tibia of the
left fore leg of a stag-beetle, affords an instructive summary of
the conditions of which we have been speaking (Fig. 121, _B_). We
see two curves. The thick one (_ab_), directed toward the axis of
the body, corresponds to the effective act of a single walking
function, which brings the body a bit forward; the thinner, on the
other hand, or we might say the hair line (_bc_), which, however, is
but rarely made quite clearly, is produced by the ineffectual
backward movement, by which the insect again approaches its working
posture (_c_). It is at first placed at some distance from the body,
in order that (like _c_ also) it may draw near to the body again;
but in such a way, naturally, that it coincides with the
starting-point of the following active curve (_cd_). It is evident
that even the passive curve is not the imprint of the movement
accomplished exclusively by the leg, for this latter, while
struggling to reach its resting-place, is really involuntarily
carried forward with the rest of the body.

“The scroll-like lines drawn by the swimming beetle (Dyticus), with
the large, sharp points of its hind tibia, are also very instructive
(Fig. 119, _A_).

FIG. 119.—_A_, trail curves described by the tibial spines of the
right and left hind limb of Dyticus. _B_, the same made by the
right hind leg (_r_{3}_) alone. Natural size.—After Graber.
]

FIG. 120.—The same by the two hind legs of Melolontha: _a_, the
active and thickened section of the curve. Natural size.
]

FIG. 121.—_A_, track curves of two of the tibial spines of the left,
middle legs of a stag-beetle. Natural size. _B_, the same
enlarged; _fg_, the longitudinal axis of the trunk; _cd_ and _ab_,
the active curve passing inward,—_bc_ and _de_, the passive going
outward. _C_, two curves described by the left hind legs; in this
case, the curves are not inwards or backwards, but partly directly
inward (_b_), and in part obliquely forwards (_a_).
]

“The diversions and modifications in the course of the active step,
as furnished by the moving factor of the remaining legs, are already
clearly illustrated by the curves shown by the joints of the hind
tibia of a May-beetle (Fig. 120) and a stag-beetle (Fig. 121, _c_).
The actual faint line in this case does not run from the front
toward the back, as would correspond to the active leg-motion, but
either directly inward (Fig. 121, _cb_), or even somewhat to the
front. In the May-beetles, and even more in the running
garden-beetle, the curves of the hind legs present themselves as
screw-like lines (Fig. 122, _l_{3}_), while the scrawling of the
remaining members (_l_{1}_, _l_{2}_) is much simpler.

“Inasmuch as we now have a cursory knowledge of the movements made
by each individual leg for itself,—movements, however, which plainly
occur very differently according to the structure of these
appendages,—the question now is of the combined play, the total
effect of all the legs taken together, and therefore of the walk and
measure of the united work of the foot.