Chapter VI: Part I: Morphology and Physiology (2)

_Development either direct (Synaptera), or with an incomplete (with nymph and winged or imaginal stages), or complete metamorphosis; in the latter case with a larval, pupal, and imago stage._

The insects may be divided into two sub-classes,—the _Synaptera_, and the winged orders, _Pterygota_, of Gegenbaur (1877), since the differences between the two groups appear on the whole to be of more than ordinal rank.

1. EXTERNAL ANATOMY

_a._ The regions of the body

The insects differ from other arthropods in that the body is divided into three distinct regions,—the head, thorax, and abdomen, the latter regions in certain generalized forms not always very distinctly differentiated. The body behind the head may also conveniently be called the trunk, and the segments composing it the trunk-segments.

In insects the head is larger in proportion to the trunk than in other classes, notably the Crustacea; the thorax is usually slightly or somewhat larger than the head, while the hind-body or abdomen is much the larger region, as it consists of ten to eleven, and perhaps in the Dermaptera and Orthoptera twelve, segments, and contains the mid- and hind-intestine, as well as the reproductive organs.

When we compare the body of an insect with that of a worm, in which the rings are distinctly developed, we see that in insects ring distinctions have given way to regional distinctions. The segments lose their individuality. It is comparatively easy to trace the segments in the hind-body of an insect, as in this region they are least modified; so with the thorax; but in the head of the adult insect it is impossible to discover the primitive segments, as they are fused together into a sort of capsule, and have almost entirely lost their individuality.

In general it may be said that the head contains or bears the organs of sense and of prehension and mastication of the food; the thorax the organs of locomotion; and the abdomen those of reproduction.

When we compare the body of a wasp or bee with that of a worm, we see that there is a decided transfer of parts headward; this process of cephalization so marked in the Crustacea likewise obtains in insects. Also the two hinder regions of the body are, in a much greater degree than in worms, governed by the brain, the principal seat of the intelligence, which, so to speak, dominates and unifies the functions of the body, both digestive, locomotive, and reproductive, as also those of the muscles moving the different segments and regions of the body. To a large extent arthropodan morphology and class distinctions are based on the regional arrangement of the somites themselves. Thus in the process of grouping of the segments into the three regions, some increase in size, while others undergo a greater or less degree of reduction; one segment being developed at the expense of one or more adjoining ones. This principle was first pointed out by Audouin, and is called Audouin’s law. It is owing to the greater development of certain segments and the reduction of others, both of the body-segments and of the segments of the limbs, that we have the wonderful diversity of form in the species and genera, and higher groups of insects, as well as those of other arthropods.

_b._ The integument (exoskeleton)

The skin or integument of insects consists, primarily, as in worms and all arthropods, of an epithelial layer of cells called the _hypodermis_. This layer secretes the cuticle, which is of varying thickness and flexibility, and is usually very dense, impermeable, and light, compared with the crust of the Crustacea, where the cuticle becomes heavy and solid by the deposition of the carbonate and phosphate of lime. This is due to the presence of a substance called by Odier chitin.[9] The cuticle is thin, delicate, and flexible between the joints; it is likewise so in such diaphanous aquatic larvæ as that of Corethra, and in the gills of aquatic insects, also in the walls of the tracheæ and of the salivary ducts. The cuticle thus forms a more or less solid crust which is broken into joints and pieces (sclerites), forming supports for the attachments of the muscles and serving to protect the soft parts within.

=Chitin.=—If we allow an insect to soak for a long time in acids, or boil it in liquid potassa or caustic potash, the integument is not affected. The muscles and the other soft parts are dissolved, leaving the cuticle clear and transparent. This insolubility of the cuticle is due to the presence of chitin, the insoluble residue left after such treatment. It also resists boiling in acids, in any alkalies, alcohol or ether. The chemical formula is C_{15}H_{26}N_{2}O_{10}.[10]

“Chitin forms less than one-half by weight of the integument, but it
is so coherent and uniformly distributed that when isolated by
chemical reagents, and even when cautiously calcined, it retains its
original organized form. The color which it frequently exhibits is
not due to any essential ingredient; it may be diminished or even
destroyed by various bleaching processes.” (Miall and Denny.)

“The chemical stability of chitin is so remarkable that we might
expect it to accumulate like the inorganic constituents of animal
skeletons, and form permanent deposits. Schlossberger (Ann. d. chem.
u. pharm., bd. 98) has, however, shown that it changes slowly under
the action of water. Chitin kept for a year under water partially
dissolved, turned into a slimy mass, and gave off a peculiar smell.
This looks as if it were liable to putrefaction. The minute
proportion of nitrogen in its composition may explain the complete
disappearance of chitin in nature.” (Miall and Denny, The Cockroach,
p. 29.)

Chitin, or a substance closely similar to it, occurs in worms and in
their tubes, especially in the pharyngeal teeth of annelids and in
their setæ. The shell of Lingula and the pen of cuttle-fish contain
true chitin (Krukenberg). The integument of Limulus, of trilobites,
and of Arachnida, as well as Myriopoda, appears to consist of
chitin.[11]

The chitin is rapidly deposited at the end of embryonic life, also during the larval and pupal stages. As is well known, insects after moulting are white, but in a few hours turn dark, and those which live in total darkness are white, showing that light has a direct effect in causing the dark color of the integument.

Moseley analyzed one pound weight of Blatta, and found plenty of iron with a remarkable quantity of manganese.

Schneider regarded chitin as a hardening of the protoplasm rather than a secretion, and the cuticle is looked upon as an exudation. It is structureless, not consisting of cells, and consists of fine irregular laminæ. “A cross-section of the chitinous layer or ‘cuticle’ examined with a high power shows extremely close and fine lines perpendicular to the laminæ.” In the cockroach the free surface of the cuticle is divided into polygonal, raised spaces or areas which correspond each to a chitinous cell of the hypodermis. (Miall and Denny.)

Numerous pore-canals pass through the cuticle of all the external parts of the body. The larger canals nearly always form the way for the passage of secretions from dermal cells, or connect with the cavities of hairs or setæ; when very fine and not connected with hairs or scales, they are either empty or filled with air, and may possibly serve for respiration.

Vosseler distinguishes in the cuticle two layers of different physical and chemical characters. Besides the external chitinous layer there is an inner layer which entirely agrees with cellulose. (Zool. Centralblatt, ii, 1895, p. 117.)

The reparative nature of chitin is seen in the fact that Verhoeff finds that a wound on an adult Carabus, and presumably on other insects, is speedily closed, not merely by a clot of blood, but by a new growth of chitin.

_c._ Mechanical origin and structure of the segments (somites,
arthromeres, metameres, zonites)

The segments are merely thickenings of the skin connected by folds or duplications of the integument, and not actually separate or individual rings or segments. This is shown by longitudinal (sagittal) sections through the body, and also by soaking or boiling the entire insect in caustic potash, when it is seen that the integument is continuous and not actually subdivided into separate somites or arthromeres, since they are seen to be connected by a thin intersegmental membrane (Fig. 16). But this segmentation or metamerism of the integument is, however, the external indication of the segmentation of the arthropodan body most probably inherited from the worms, being a disposition of the soft parts which is characteristic of the vermian type. This segmentation of the integument is correlated with the serial repetition of the ganglia of the nervous system, of the ostia of the dorsal vessel, the primitive disposition of the segmental and reproductive organs, of the soft, muscular dissepiments which correspond to the suture between the segments, and with the metameric arrangement of the muscles controlling the movements of the segments on each other, and which internal segmentation or metamerism is indicated very early in embryonic life by the mesoblastic somites.

FIG. 16.—Diagram of the anterior part of an insect, showing the
membranous intersegmental folds, _g_.—After Graber.
]

In the unjointed worms, as Graber states, the body forms a single but flexible lever. In the earthworm the muscular tube or body-wall is enclosed by a stiffer cuticle, divided into segments; hence the worm can move in all required directions, but only by sections, as seen in Fig. 16, which represents the thickened integument divided into segments, and folded inward between each segment, this thin portion of the skin being the intersegmental fold. Each segment corresponds to a special zone of the subdivided muscular tube (_m_), the fascia extending longitudinally. The figure shows the mode of attachment of the fascia of the muscle-tube to the segment. The anterior edge is inserted on the stiff, unyielding, inner surface of each segment: the hinder edge of the muscle is attached to the thin, flexible, intersegmental fold, which thus acts as a tendon on which the muscle can exert its force. (Graber.)

FIG. 17.—Diagram of the integument and arrangement of the segmental
muscles: _A_, relaxed; _m_, muscle; _g_, membranous articulation;
_r_, chitinous ring. _B_, the same contracted on both sides. _C_, on
one side.—After Graber.
]

“Fig. 17 makes this still clearer. The muscles (_m_) extend between two segments immediately succeeding each other. Supposing the anterior one (_A_) to be stationary, what do we then see when the muscle contracts? Does it also become shorter? The intersegmental fold is drawn forwards, and hence the entire hinder segment moves forward and is shoved into the front one, and so on with the others, as at _B_. Afterwards, if the strain of the muscle is relieved by the diminishing action of the tensely stretched, intersegmental membrane, it again returns to a state of rest.” (Graber.)

FIG. 18.—Diagrams to demonstrate the mechanism of the motion of the
segmented body in the Arthropoda: One larger segment (_cf_) and 4
smaller. The exoskeleton is indicated by black lines, the
interarticular membranes by dotted lines. The hinges between
consecutive segments are marked _at_, tergal (dorsal) skeleton; _s_,
sternal (ventral) skeleton; _d_, dorsal longitudinal muscles =
extensors (and flexors in an upward direction); _v_, ventral
longitudinal muscles = flexors. In _B_, the row of segments is
stretched; in _A_, by the contraction of the muscles (_d_) bent
upward; in _C_, downward; _tg_, tergal; _sg_, sternal interarticular
membranes.—After Lang.
]

While we look upon the dermal tube of worms as a single but flexible lever, the body of the arthropods, as Graber states, is a linear system of stiff levers. We have here a series of stiff, solid rings, or hooks, united by the intersegmental membrane into a whole. When the muscles, extending from one ring to the next behind contract, and so on through the entire series, the rings approximate each other.

The ectoskeletal segments bend to one side by the contraction of the muscles on one side, the point of the outer segmental fold opposite the fixed point becoming converted into the turning-point (_C_).

The usual result of the arrangement of the locomotive system is the simple curving of the body (_C_), and then the alternate bending of the body to right and left, which produces the serpentine movements characteristic of the earthworms, the centipede, and many insect larvæ. The most striking example of the wonderful variety of movements which can be made by an insect are those of the Syrphus larva. When feeding amid a herd of aphides, it is seen to now raise the front part of the body erect and stiff, then to bend it down, or rapidly turn it to either side, or move it in a complete circle. (Graber, pp. 23–26.)

The arrangement and mode of working of the muscles, says Lang, is illustrated by Fig. 18, which shows us five segments, one larger (_ct_) and four smaller, in vertical projection. The thicker portion of the integument is marked by strong outlines, the delicate and flexible interarticular membranes (_tg_, _sg_) in dotted lines. The hinges between two consecutive segments are marked _a_. A dorsal muscle (_d_) is attached to the larger segment (_ct_), and runs through the smaller segments, being inserted in the dorsal portion of the crust (_t_) of each by means of a bundle of fibres. A ventral muscle (_v_) does the same on the sternal side (_s_).

“The skeletal segments,” adds Lang, “may be compared to a double-armed lever, whose fulcrum lies in the hinges. If the dorsal muscle contracts, it draws the dorsal arm of the lever (the tergal portion of the skeleton) in the direction of the pull towards the larger segments; the tergal interarticular membranes become folded, the ventral stretched, and the four segments bend upward (Fig. 18, _A_). If the ventral muscle contracts, while at the same time the dorsal slackens, the row of segments will be bent downwards (Fig. 18, _C_).”

L. B. Sharp suggests, that in the Crustacea the rings formed by “the regularity and stress of muscular action” would be hardened by the deposition of lime at the most prominent portion, _i.e._ between what we have called the intersegmental folds. (American Naturalist, 1893, p. 89.) Cope also states that “with the beginning of induration of the integument, segmentation would immediately appear, for the movements of the body and limbs would interrupt the deposit at such points as would experience the greatest flexure. The muscular system would initiate the process, since flexure depends on its contractions, and its presence in animals prior to the induration of the integuments in the order of phylogeny, furnishes the conditions required.” (The Primary Factors of Organic Evolution, p. 268, 1895.)

It is apparent that the jointed or metameric structure of the bodies of insects and other arthropods is an inheritance from the segmented worms. In the worms the body is a continuous dermo-muscular tube, while in arthropods this tube is divided into regions, and the cuticle is thicker and more resistant. To go back to the incipient stages in the process of segmentation of the body, we conceive that the worms probably arose from a creeping gastrula-like form, the gastræa. The act of creeping gradually induced an elongated shape of the body. The movement of such an organism in a forward direction would gradually evolve a fore and aft, dorsal and ventral, and bilateral symmetry. As soon as this was attained, as the effect of creeping over rough irregular surfaces there would result mechanical lateral strains intermittently acting during the serpentine movements of the worm. The integument would, we can readily suppose, tend to bend or yield, or become permanently wrinkled, at more or less regular intervals. The arrangement of the muscles would gradually conform to this habit of creeping, and finally the nervous system and other organs more directly connected with the creeping movements of the organism would tend to be correlated in their arrangement with that of the segments. In this way the homonomous segments of the annelid body probably became developed, and their relations and shapes were eventually fixed by inheritance. After this stage was reached, and limbs began to appear, the segments would tend to become heteronomous, and to be grouped into regions.

FIG. 19.—_Dujardinia rotifera_, with jointed tentacles and caudal
appendages.—With some changes, after Quatrefages.
]

The origin of the joints or segments in the limbs of arthropods was probably due to the mechanical strains to which what were at first soft fleshy outgrowths along the sides of the body became subjected. Indeed, certain annelid worms of the family Syllidæ have segmented tentacles and parapodia, as in Dujardinia (Fig. 19). We do not know enough about the habits of these worms to understand how this metamerism may have arisen, but it is possibly due to the act of pushing or repeated efforts to support the body while creeping over the bottom among broken shells, over coarse gravel, or among seaweeds.

It is obvious, however, that the jointed structure of the limbs of arthropods, if we are to attempt any explanation at all of the origin of such structure, was primarily due mainly to lateral strains and impacts resulting from the primitive endeavors of the ancestral arthropods to raise and to support the body while thus raised, and then to push or drag it forward by means of the soft, partially jointed, lateral limbs which were armed with bristles, hooks, or finally claws.

On the other hand, by adaptation, or as the result of parasitism and consequent lack of active motion, the original number of segments may by disuse be diminished. Thus in adult wasps and bees, the last three or four abdominal segments may be nearly lost, though the larval number is ten. During metamorphosis the body is made over, and the number, shape, and structure of the segments greatly modified. In the female of the Stylopidæ the thorax loses all traces of segments, and is fused with the head, and the abdominal segments are faintly marked, losing their chitin.

While the maxillæ have several joints, the mandibles are 1–jointed, but there are traces of two joints in Campodea, certain beetles, etc. In the antenna there is a great elasticity in respect to the number of joints, which vary from one or two to a hundred or more. It is likewise so in the thoracic legs, where the number of tarsal joints varies from one to five; also in the cercopoda, the number of joints varying from one or two to twelve or more.

_d._ Mechanical origin of the limbs and of their jointed structure

We have already hinted at the mode of origin of the limbs of arthropods. Like the body or trunk, the limbs are chitinous dermo-muscular tubes, with a dense solid cuticle, and internal muscles, and were it not for their division at more or less regular intervals into segments, forming distinct sets of levers, set up by the strains in these tubular supports, there would be no power of varied motion.

Even certain worms, as already stated, have their tentacles and parapodia, or certain appendages of their parapodia, more or less jointed, but there are no indications of claws or of any other hard chitinous armature at the extremity, and the skin is thin and soft.

In the most simple though not the most primitive arthropods, such as the Tardigrades, whose body is not segmented, there are four pairs of short unjointed legs, ending each in two claws, which have probably arisen in response to the stimulus of pushing or dragging efforts.

The legs of Peripatus are unjointed, and have a thin cuticle, but end in a pair of claws, which have evidently arisen as a supporting armature, the result of the act of moving or pulling the body over the uneven surface of the ground.

FIG. 20.—A prothoracic leg of Chironomus larva; and pupa.
]

FIG. 21.—_A_, larva of _Ephydra californica_: _a_, _b_, _c_, pupa.
]

There is good reason to suppose that such limbs arose from dynamical causes, similar to those exciting the formation of secondary adaptations such as are to be seen in the prop or supporting legs of certain dipterous larvæ, as the single pair of Chironomus (Fig. 20) and Simulium, or the series of unjointed soft tubercles of Ephydra (Fig. 21), etc., which are armed with hooks and claws, and are thus adapted for dragging the insect through or over vegetation or along the ground.

Now by frequent continuous use of such unjointed structures, the cuticle would tend to become hard, owing to the deposit of a greater amount of chitin between the folds of the skin, until finally the body being elongated and homonomously segmented, the movements of walking or running would be regular and even, and we would have homonomously jointed legs like those of the trilobites, or of the most generalized Crustacea and of Myriopoda.

In the most primitive arthropods,—and such we take it were on the whole the trilobites, rather than the Crustacea,—the limbs were of nearly the same shape, being long and slender and evenly jointed from and including the antennæ, to the last pair of limbs of the abdominal region. In these forms there appear to be, so far as we now know, no differentiation into mandibles, maxillæ, maxillipedes, and thoracic legs, or into gonopoda. The same lack of diversity of structure and function of the head-appendages has survived, with little change, in Limulus. In the trilobites (Fig. 1) none of the limbs have yet been found to end in claws or forceps; being in this respect nearly as primitive as in the worms. Secondary adaptations have arisen in Limulus, the cephalic appendages being forcipated, adapted as supports to the body and for pushing it onward through the sand or mud, while the abdominal legs are broad and flat, adapted for swimming and bearing the broad gill-leaves.

It is thus quite evident that we have three stages in the evolution of the arthropodan limb; _i.e._ 1, the syllid stage, of simple, jointed, soft, yielding appendages not used as true supports (Fig. 19); 2, the trilobite stage, where they are more solid, evenly jointed, but not ending in claws; and by their comparatively great numbers (as in the trilobite, Triarthrus) fully supporting the body on the bottom of the sea. In Limulus they are much fewer in number, thicker, and acting as firm supports, the cephalic limbs of use in creeping, and ending in solid claws. 3, The third stage is the long slender swimming head-appendages of the nauplius stage of Crustacea.

As regards the evolution of limbs of terrestrial arthropods, we have the following stages: 1, the soft unjointed limbs of Tardigrades, ending in two claws, and those of Peripatus, and the pseudo- or prop-legs of certain dipterous larvæ; 2, finally the evolution of the long, solid, jointed limbs of Pauropus and other primitive myriopods, the legs forming solid, firm supports elevating the body, and enabling the insect to drag itself over the ground or to walk or run. When the body is elongated and many-segmented, the legs are necessarily numerous; but when it is short, the legs become few in number, _i.e._ six, in the hexapodous young of myriopods and in insects, or eight in Arachnida. Whenever the legs are used for walking, _i.e._ to raise and support the body, they end in a solid point or in a pair of forceps or claws. On the other hand, as in phyllopods, where the legs are used mainly for swimming, they are unarmed and are soft and membranous, or, as in the limbs of the nauplius or zoëa stage of crustaceans, end in a simple soft point, which often bears tactile setæ.

The tarsal joints are more numerous in order to give greater flexibility to the limb in seizing and grasping objects, both to drag the body forwards and to support it.

Unlike those of the Crustacea, the limbs of insects are not primitively biramose, but single, the three-lobed first maxillæ, and secondarily bilobed second maxillæ being the result of adaptation. Embryology on the whole proves the truth of this assumption; the maxillæ of both pairs are at first single buds, afterwards becoming lobed. All the appendages of the body, including the ovipositor or sting, are modified limbs, as shown by their embryological development.

It is noticeable that in the crab, where the body is raised by the limbs above the bottom, it is much shorter and more cephalized than in the shrimps. Also in the simply walking and running spiders, the hind-body is shorter than in scorpions, while in the running and flying insects, such as the Cicindelidæ, and in the swiftly flying flies and bees, there is a tendency to a shortening of the body, especially of the abdomen. The long body of the dragon-fly is an impediment to flight, but compensated for by the action of the large wings.

The arthropodan limb is a compound leverage system. It is, says Graber, a lateral outgrowth of the trunk, which repeats in miniature that of the main trunk, its single series of joints or segments forming a jointed dermo-muscular tube. Yet the lateral appendages of an insect differ from the main trunk in two ways: (1) they taper to the end which bears the two claws, and (2) their segments are in the living animal arranged not in a straight line, but at different angles to each other. The basal joint turning on the trunk acts as the first of a whole series of levers. The second joint, however, is connected with the musculature of the first or basal joint, and thus each succeeding joint is moved on the one preceding. Each lever, from the first to the last, is both an active and a passive instrument. (Graber.)

While, however, as Graber states, the limbs possess their own sets of muscles and can move by the turning of the basal joint, the labor is very much facilitated, as is readily seen, by the trunk, though the latter has to a great extent delegated its locomotive function to the appendages, which again divide its labor among the separate joints.

Graber then calls attention to the analogy of the mechanics of locomotion of insects to those of vertebrates. An insect’s and a vertebrate’s legs are constructed on the same general mechanical principles, the limbs of each forming a series of levers.

FIG. 22.—Diagram of the knee-joint of a vertebrate (_A_) and an
insect’s limb (_B_): _a_, upper; _b_, lower, shank, united at _A_ by
a capsular joint, at _B_ by a folding joint; _d_, extensor or
lifting muscle; _d_^1, flexor or lowering muscle of the lower joint.
The dotted line indicates in _A_ the contour of the leg.—After
Graber.
]

Fig. 22, _A_, represents diagrammatically the knee joint of a vertebrate, and _B_ that of an insect; _a_, the femur or thigh, and _b_, the tibia or shank. In the vertebrate the internally situated bones are brought into close union and bend by means of a hinge-joint; so also in the chitinous-skinned insect.

The stiff dermal tube of the insect acts as a lever by means of the thin intersegmental membrane (_c_) pushed in or telescoped in to the thigh joint, a special joint-capsule being superfluous. The muscles are in general the same in both types; they form a circle. In both the shank is extended by the contraction of the upper muscles (_d_) and is bent by the contraction of the lower (_d^1_). The intersegmental membrane of the insect’s limb is in a degree a two-armed lever, whose pivot (_f_) lies in the middle. The internal invagination of the intersegmental fold (_B_, _g-h_) affords the necessary support to the muscles acting like the tendon in the vertebrate. (Graber.)

FIG. 23.—Primitive band or germ of a Sphinx moth, with the segments
indicated, and their rudimentary appendages: _c_, upper lip; _at_,
antennæ; _md_, mandibles; _mx_, _mx′_, first and second maxillæ;
_l_, _l′_, _l″_, legs; _al_, abdominal legs.—After Kowalevsky.
]

Graber also calls attention to the fact that this insect limb differs in one important respect from that of land vertebrates. The leverage system in the last is divided at the end into five parallel divisions or digits. In arthropods, on the contrary, all the joints succeed one another in a linear series.

In insects, as well as in other arthropods, modifications of the limbs usually take the form of a simple reduction in the number of segments. Thus while the normal number of tarsal joints is five, we have trimerous and dimerous Coleoptera, and in certain Scarabæidæ the anterior tarsi are lost.

Savigny was the first, in 1816, in his great work, “Théorie des organes de la bouche des Crustacés et des Insectes,” to demonstrate that not only were the buccal appendages of biting insects homologous with those of bugs, moths, flies, etc., but that they were homologous with the thoracic legs, and that thus a unity of structure prevails throughout the appendages of the body of all arthropods. Oken also observed that “the maxillæ are only repeated feet.”

What was modestly put forth as a theory by the French morphologist has been abundantly proved by the embryology of insects of different orders to be a fact. As shown in Fig. 23 the antennæ and buccal appendages arise as paired tubercles exactly as the thoracic legs. The abdominal region also bears similar embryonic or temporary limbs, all of which in those insects without an ovipositor disappear, except the cercopoda, after birth.

LITERATURE ON THE EXTERNAL ANATOMY

General

=Swammerdam, Johann.= Biblia naturæ. (In Dutch, German, and English.)
1737–1738, fol., London, 1758, Pls.

=Réaumur, Réné Antoine Ferchault, de.= Mémoires pour servir à
l’histoire des insectes. i-iv, 4º, Paris, 1734–1742.

=Lyonet, Pieter.= Traité anatomique de la chenille, qui ronge le bois
de saule, etc. 4º, pp. xxii, 616. À la Haye, 1732. Tab. 18.

—— Recherches sur l’anatomie et les metamorphoses de differentes
espèces d’insectes. Ouvrage posthume, publié par M. W. de Haan. pp.
580, tab. 54, 1832.

=Latreille, Pierre André.= Des rapports généraux de l’organization
extérieure des animaux invertébres articulés, et comparaison des
Annelides avec les Myriapodes. (Mémoires du Mus. d’Hist. Nat., 1820,
vi, pp. 116–144.)

—— De quelques appendices particuliers du thorax de divers insectes.
(Mémoires du Mus. d’Hist. Nat., 1821, vii, pp. 1–21).

—— Observations nouvelles sur l’organization extérieure et générale
des animaux articulés et à pieds articulés, et application de ces
connoissances à la nomenclature des principales parties des mêmes
animaux. (Mèmoires du Mus. d’Hist. Nat., viii, 1822, pp. 169–202.)

=Cuvier, George Leopold Christian Dagobert.= Rapport sur les
recherches anatomiques sur le thorax des animaux articulés et celui
des insectes en particulier par M. V. Audouin. 4º, pp. 15, tab. 1,
Paris, 1823.

=Audouin, Jean Victor.= Recherches anatomiques sur le thorax des
animaux articulés et celui des insectes hexapodes en particulier.
(Annales des Sciences naturelles, i, pp. 97–135, 416–432, 1824.)

=Kirby, William, and William Spence.= Introduction to entomology.
i-iv, 1816–1828, London.

=Straus-Durckheim, Hercule.= Considérations générales sur l’anatomie
comparée des animaux articulés. Paris, 1828, atlas of 19 plates.

=MacLeay, William Sharp.= Explanation of the comparative anatomy of
the thorax in winged insects, with a review of the present state of
the nomenclature of its parts. (Zoöl. Journal, v, pp. 145–179, 1830,
2 Pls.)

=Burmeister, Hermann.= A manual of entomology. Trans. by W. E.
Shuckard, 8º, pp. 654, London, 1836, 32 Pl.

=Westwood, John Obadiah.= An introduction to the modern classification
of insects, i, ii, 8º, pp. 462, 587, 158, 1 Pl. and 133 blocks of
figs., 1839–1840.

=Newport, George.= Art. Insecta in Todd’s Cyclopædia of Anatomy and
Phys. ii, pp. 853–994, 1839, Figs. 329–439.

=Erichson, Wilhelm Ferdinand.= Entomographien. Berlin, 1840.

=Brullé, Auguste.= Recherches sur les transformations des appendices
dans les articulés. (Annales des Sciences nat. Sér. 3, ii, pp.
271–374, tab. 1, 1844.)

=Winslow, A. P.: son.= Om byggnaden af thorax hos Insekterna.
Helsingborg, 1862, 1 Pl, pp. 24.

=Packard, Alpheus Spring.= Guide to the study of insects. 1869.

—— Systematic position of the Orthoptera in relation to other insects.
(Third report U. S. Ent. Commission, pp. 286–345, 1883, Pls.
xxiii-lxi.)

=Graber, Vitus.= Die Insekten. 12º, pp. 403, 603, München, 1877, many
Figs.

=Huxley, Thomas Henry.= A manual of the anatomy of invertebrated
animals. 12º, pp. 397–451, Figs., London, 1877.

=Hammond, Arthur.= Thorax of the blow-fly. (Journ. Linn. Soc., London,
xv. Zoöl., 1880, pp. 31.)

=Brauer, Friedrich.= Ueber das Segment médiaire Latreille’s. (Sitzb.
d. k. Akad. d. Wissensch. Wien, 1882, pp. 218–241, 3 tab.)

—— Systematisch-zoologische Studien. (Ibid., 1885, pp. 237–413.)

=Gosch, C. C. A.= On Latreille’s theory of “Le Segment médiaire.”
(Nat. Tidsskrift (3), xiii, pp. 475–531, 1883.)

=Miall, L. C., and Denny, Alfred.= The structure and life-history of
the cockroach (_Periplaneta orientalis_). An introduction to the
study of insects. 8º, pp. 224, London, 1886.

=Cheshire, Frank R.= Bees and bee-keeping. i, Scientific, London,
1886, Pls. and Figs.

=Lang, Arnold.= Text-book of comparative anatomy. i, pp. 426–508,
1891, many Figs.

=Kolbe, H. J.= Einführung in die Kenntniss der Insekten. 8º, pp. 709,
324 figs., Berlin, 1893.

=Sharp, David.= The Cambridge natural history. Insecta, i, 8º, pp.
83–584, 1895, Figs. 47–371.

Also the works of Bos, Chabrier, Cholodkowsky, Comstock, Dewitz, Eaton, Erichson, Gerstaecker, Girard, Grassi, Hagen, Haase, Kellogg, Knoch, Lacordaire, Latreille, Leuckart, Lendenfeld, Lowne, Lubbock, Mayer, Meinert, F. Müller, Osten-Sacken, Pagenstecher, Reinhard, Schaum, Schiödte, Scudder, J. B. Smith, Spinola, Stein, Weismann, Wood-Mason.

THE HEAD AND ITS APPENDAGES

_a._ The head

FIG. 24.—Presumed larva of Nemoptera (_Necrophilus arenarius_),
Pyramids of Egypt.—After Roux, from Sharp.
]

While the head is originally composed of probably not less than six segments, these are in the adult insect fused together into a capsule or hard chitinous box, the _epicranium_, with no distinct traces of the primitive segments. The head contains the brain and accessory ganglia, the mouth or buccal cavity, also the air-sacs in many winged forms, and gives support to the external organs of sense, the antennæ, and to the buccal appendages, the larger part of the interior being filled with the muscles moving these structures. The solid walls of the head serve as a lever or support for the attachment of these muscles, especially those of the mandibles. Thus there is a correlation between the large size of the mandibles of the soldier white ants and ants, the head being correspondingly large to accommodate the great mandibular muscles. The other extreme is seen in the larva of Necrophilus (Fig. 24), with its long slender neck and diminutive head.

=The clypeus.=—This is that part of the head situated in front of the epicranium, and anterior to the eyes, forming the roof of the posterior part of the mouth, and is, as embryology shows, probably a tergal sclerite. It varies greatly in shape and size in the different orders of insects. It is often divided into two parts, the _clypeus posterior_ and _clypeus anterior_, or which may be designated as the _post-_ and _ante-clypeus_ (Figs. 29, _B_).

=The labrum.=—The “upper lip” or labrum is an unpaired flap-like piece hinged to the front edge of the clypeus, and may be seen to move up and down when the insect moves its mandibles. It forms the roof of the anterior part of the mouth (Figs. 69, 74), and its inner side is lined with a soft membrane, usually provided with hairs and sense-papillæ or cups, forming the epipharynx.

The labrum is more or less deeply bilobed, especially in caterpillars and in adult Staphylinidæ, and has been thought by some writers (Kowalevsky, Carrière, and also Chatin) to represent a pair of appendages, but Heymons (1895) refutes this view, stating as his reason that the labrum arises between the two halves of the nervous system (protocerebrum), while all the true appendages arise on each side of the nervous system. (See also Fig. 34.)

FIG. 25.—Front view of the head of _C. spretus_: _E_, epicranium; _C_,
clypeus; _L_, labrum; _O_, _o_, ocelli; _e_, eye; _a_, antenna;
_md_, mandible; _mx_, portion of maxilla uncovered by the labrum;
_p_, maxillary palpus; _p′_, labial palpus.
]

In the fleas (Siphonaptera) both the clypeus and labrum are wanting.

While it apparently forms an anterior specialized portion of the procephalic lobes, Viallanes regarded it as belonging to the third, or his tritocerebral, segment, since the labral nerves arise from the tritocerebral ganglia. But since in all the early as well as late stages of embryonic life it appears to be situated in front of the mouth, it would seem to belong to the first segment.

In the embryo of Blatta it first appears as a thick crescentic fold being slightly divided anterior to the mouth, and in Doryphora it appears as a heart-shaped or deeply bilobed prominence situated in front of the mouth (Wheeler).

=The epipharynx and labrum-epipharynx.=—The epipharynx is the under surface or pharyngeal lining of the clypeus and labrum, forming the membranous roof of the mouth. As it contains the organs of taste and has been generally overlooked by entomologists, we may dwell at some length on its structure in different orders.

Réaumur was, so far as we have been able to ascertain, the first author to describe and figure the epipharynx, which he observed in the honey bee and bumble bee, and called _la langue_, remarking that it closes the opening into the œsophagus, and that it is applied against the palate. According to Kirby and Spence, De Geer described the epipharynx of the wasp; and Latreille referred to it, calling it the _sous labre_.

The name _epipharynx_ was bestowed upon this organ by Savigny, who thus speaks of that of the bees: “Ce pharynx est, à la vérité, non seulement caché par la lèvre supérieure, mais encore exactement recouvert par un organe particulier que Réaumur a déjà décrit. C’est une sorte d’appendice membraneux qui est reçu entre les deux branches des mâchoires. Cette partie ayant pour base le bord supérieur du pharynx, peut prendre le nom d’_épipharynx_ ou d’_épiglosse_.”

He also describes that of Diptera. What Walter has lately proved to be the epipharynx of Lepidoptera was regarded by Savigny and all subsequent writers as the labrum.

The latest account of the function of this organ is that by Cheshire, who states that the tube made by the maxillæ and labial palpi cannot act as a suction pipe, because it is open above. “This opening is closed by the front extension of the epipharynx, which closes down to the maxillæ, fitting exactly into the space they leave uncovered, and thus the tube is completed from their termination to the œsophagus.”

FIG. 26.—Epipharynx of _Phaneroptera angustifolia_: _cl_, clypeus;
_lbr. e_, labrum-epipharynx; _t_ _c_, taste cups, both on the
clypeal and on the labral regions.
]

FIG. 27.—Epipharynx of _Hadenœcus subterraneus_, cave cricket.
]

It is singular that this organ is not mentioned in Burmeister’s Manual of entomology, in Lacordaire’s Introduction à l’entomologie, or by Newport in his admirable article _Insecta_ in Todd’s Cyclopedia of anatomy. Neither has Straus-Durckheim referred to or figured it in his great work on the anatomy of _Melolontha vulgaris_.

In their excellent work on the cockroach, Miall and Denny state that “The epipharynx, which is a prominent part in Coleoptera and Diptera, is not recognizable in Orthoptera” (p. 45). We have, however, found it to be always present in this order (Figs. 26, 27).

We are not aware that any modern writers have described or referred to the epipharynx of the mandibulate orders of insects. Although Dr. G. Joseph speaks of finding taste-organs on the palate of almost every order of insects, especially plant-feeding forms, we are unable to find any specific references, his detailed observations being apparently unpublished.

The epipharynx is so intimately associated with the elongated labium of certain Diptera, that, with Dr. Dimmock, we may refer to the double organ as the labrum-epipharynx; and where, as in the lepidopterous _Micropteryx semipurpurella_, described and figured by Walter, and the Panorpidæ (Panorpa and Boreus), the labrum seems pieced out with a thin, pale membranous fold which appears to be an extension of the epipharynx, building up the dorsal end of the labrum, this term is a convenient one to use.

In the lower orders of truly mandibulate insects, from the Thysanura to the Coleoptera, excluding those which suck in liquid food, such as the Diptera, Lepidoptera, and Hymenoptera, and the Mecoptera (Panorpidæ) with their elongated head and feeble, small mandibles, the epipharynx forms a simple membranous palatal lining of the clypeus and labrum. In such insects there is no soft projecting or pendant portion, fitted to close the throat or to complete a partially tubular arrangement of the first and second maxillæ.

In all the mandibulate insects, then, the epipharynx forms simply the under surface or pharyngeal lining of the clypeus and labrum, the surface being uniformly moderately convex, and corresponding in extent to that of the clypeus and labrum, posteriorly merging into the palatal wall of the pharynx; the armature of peculiar gathering-hairs sometimes spreading over its base, being continuous with those lining the mouth and beginning of the œsophagus. The suture separating the labrum from the clypeus does not involve the epipharynx, though since certain gustatory fields lie under the front edge of the clypeus, as well as labrum, one may in describing them refer to certain fields or groups of cups or pits as occupying a labral or clypeal region or position.

The lack of traces of a suture in the epipharynx corresponding to the labral suture above, suggests that the labrum does not represent a pair of coalesced appendages, and that it, with the clypeus, simply forms the solid cuticular roof of the mouth.

The only soft structures seen between the epipharynx and labrum, besides the nerves of special sense, are the elevator muscles of the labrum, and two tracheæ, one on each side.

The structure and armature of the epipharyngeal surface even besides the taste-pits, taste-cups and rods, is very varied, the setæ assuming very different shapes. There seem to be two primary forms of setæ, (1) the normal forms which arise from a definite cell; and (2) soft, flattened, often hooked hairs which are cylindrical towards the end, but arise from a broad triangular base, without any cell-wall. These are like the “gathering hairs” of Cheshire, situated on the bees’ and wasps’ tongue; they also line the walls of the pharynx and extend toward the œsophagus. They are also similar to the “hooked hairs” of Will. The first kind, or normal setæ, are either simply defensive, often guarding the sense-cups or sensory fields on which the sense-cups are situated, or they have a nerve extending to them and are simply tactile in function.

The surface of the epipharynx, then, appears to be highly sensitive, and to afford the principal seat of the gustatory organs, which are described under the head of organs of taste.

LITERATURE ON THE EPIPHARYNX

=Réaumur.= Mémoires pour servir à l’histoire des insectes, v, 1740, p.
318, Pl. 28, Figs. 4, 7, 8, 9, 10, 11 l.

=Kirby and Spence.= Intr. to entomology, iii, 1828, p. 457.

=De Geer.= ii, 1778; v, 26, Fig. 11, M.

=Kirby and Spence.= Pl. xii, Fig. 2 K.

=Latreille.= Organisation extérieure des insectes, p. 184. (Quoted
from Kirby and Spence.)

=Savigny.= Mémoires sur les animaux sans vertèbres. Partie I^{re},
1816, p. 12.

=Walter, Alfred.= Beiträge zur Morphologie der Schmetterlinge. Erster
Theil. Zur Morphologie des Schmetterlingsmundtheile. (Jena. Zeits.,
xviii, 1885, p. 752.)

=Cheshire, F. R.= Bees and bee-keeping, i, London, 1886, p. 93.

=Joseph, Gustav.= Zur Morphologie des Geschmacksorganes bei Inseckten.
(Amtlicher Bericht der 50 Versammlung deutscher Naturforscher u.
Artzte in München. 1877, pp. 227, 228.)

=Dimmock, George.= The anatomy of the mouth-parts and of the sucking
apparatus of some Diptera, 1881. (Also in Psyche, iii, pp. 231–241,
Pl. 1, 1882.)

=Packard, A. S.= On the epipharynx of the Panorpidæ. (Psyche, 1889, v,
pp. 159–164.)

—— Notes on the epipharynx and the epipharyngeal organs of taste in
mandibulate insects. (Psyche, v, pp. 193–199, 222–228, 1889.)

=Attachment of the head to the trunk.=—The head is either firmly supported by the broad prothoracic segment in Orthoptera, many beetles, etc., into which it is more or less retracted, or it is free and attached by a slender neck, easily turning on the trunk, as in dragon-flies, flies, etc. In some insects there are several chitinous plates, situated on an island in the membrane on the under side of the neck; these are the “cervical sclerites” of Sharp, occurring “in Hymenoptera, in many Coleoptera, and in Blattidæ.”

=The basal or gular region of the head.=—At the hinder part of the head is the opening (occipital foramen) into the trunk. The cheek (gena) is the side of the head, and to its inner wall is attached the mandibular muscle; it thus forms the region behind the eye and over the base of the mandibles. In the Termitidæ, where the head is broad and flat, it forms a distinct piece on the under side of the head bounding the gulo-mental region (Fig. 28). In the Neuroptera (Corydalus, Fig. 29, and Mantispa, Fig. 30) it is less definitely outlined.

FIG. 29.—Head of _Corydalus cornutus_, ♂: _A_, from above. _B_, from
beneath. _C_, from the side. _a. cly_, clypeus anterior; _p. cly_,
clypeus posterior; _lbr_, labrum; _md_, mandible; _mx_, base of
first maxilla; _mp_, its palpus; _m_, mentum; _sm_, submentum;
_plpr_, palpifer; _lig_, fused second maxillæ; _ant_, antenna;
_occ_, occiput.
]

FIG. 28.—Head of _Termopsis angusticollis_, seen from beneath, showing
the gena and gula: _m_, mentum; _sm_, submentum; _labr_, under side
of the labrum; _x_, hypopharyngeal chitinous support.
]

All the gulo-mental region of the head appears to represent the base of the second maxillæ, and the question hence arises whether the submentum is not the homologue of the cardines of the first maxillæ fused, and the mentum that of the stipites of the latter also fused together. If this should prove to be the case, the homologies between the two pairs of maxillæ will be still closer than before supposed. Where the gula is differentiated, this represents the basal piece of the second maxillæ. In Figs. 28, 29, 30, and 31, these three pieces are clearly shown to belong to the second maxillary segment. It is evident that these pieces or sclerites belong to the second maxillary or labial segment of the head, as does the occiput, which may represent the tergo-pleural portion of the segment. Miall and Denny also regarded the submentum as the basal piece of the second maxillæ.

FIG. 30.—Head of _Mantispa brunnea_, under side: _e_, eye; other
lettering as in Fig. 29.
]

FIG. 31.—Head of _Limnephilus pudicus_, under side: _e_, eye; _l_,
ligula; _p_, palpifer; _lp_, labial palpi.
]

=The occiput= (Fig. 29, _B_, _C_), as stated beyond, is very rarely present as a separate piece; in the adult insect we have only observed it in Corydalus. The occipital region may be designated as that part of the head adjoining and containing the occipital foramen. Newport considers the occiput as that portion of the base of the head “which is articulated with the anterior margin of the prothorax. It is perforated by a large foramen, through which the organs of the head are connected with those of the trunk. It is very distinct in Hydroüs and most Coleoptera, and in some, the Staphylinidæ, Carabidæ, and Silphidæ is constricted and extended backwards so as to form a complete neck.” (See also p. 51.)

FIG. 32.—Interior and upper and under surface of the head of _Hydroüs
piceus_: _d_, clypeus; _e_, labrum; _g_, maxilla; _h_, its palpus;
_i_, labium; _k_, labial palpus; _p_, sutura epicranii; _q_,
cotyloid cavity; _r_, torulus; _s_, _v_, laminæ squamosa; _t_,
laminaæ posteriores; _u_, tentorium; _w_, laminæ orbitales; _x_, os
transversum; _y_, articulating cavity for the mandible; _z_, os
hypopharyngeum.—After Newport.
]

=The tentorium.=—The walls of the head are supported or braced within by two beams or endosternites passing inwards, and forming a solid chitinous process or loop which extends in the cockroach downwards and forwards from the lower edge of the occipital foramen. “In front it gives off two long crura or props, which pass to the ginglymus, and are reflected thence upon the inner surface of the clypeus, ascending as high as the antennary socket, round which they form a kind of rim.” (Miall and Denny.) The œsophagus passes upwards between its anterior crura, the long flexor of the mandible lies on each side of the central plate; the supraœsophageal ganglion rests on the plate above, and the subœsophageal ganglion lies below it, the nerve cords which unite the two passing through the circular aperture. (Miall and Denny.) In Coleoptera (Hydroüs) it protects the nervous cord which passes under it. (Newport, Fig. 32, _u_.)

FIG. 33.—Posterior view of head of Anabrus; _t_, tentorium. Joutel
_del._
]

In Anabrus the tentorium is V-shaped, the two arms originating on each side of the base of the clypeus next to the base of each mandible the origin being indicated by two small foramina partly concealed externally and passing inwards and backwards and uniting just before reaching the posterior edge of the large occipital foramen (Fig. 33).

Palmén regards the tentorium as representing a pair of tracheæ (with
the cephalic spiracles) which have become modified for supports or
for muscular attachment, since he finds that in Ephemera the
tentorium breaks across the middle during exuviation, each half
being drawn out of the head like the chitinous lining of a tracheal
tube. This view is supported by Wheeler, who has shown that the
tentorium of Doryphora originates from five pairs of invaginations
of the longitudinal commissures, and which are anterior to those of
the second maxillary segment. “These invaginations grow inwards as
slender tubes, which anastomose in some places. Their lumina are
ultimately filled with chitin.” (Jour. Morph., iii, p. 368.)

This view has also been held by Carrière and Cholodkowsky, but
Heymons concludes from his embryological studies on Forficula and
Blattidæ (1895) that it is unfounded. That this is probably the case
is proved by the fact that the apodemes of the thoracic region are
evidently not modified tracheæ, since the stigmata and tracheæ are
present.

=Number of segments in the head.=—While it is taken for granted by many entomologists that the head of insects represents a single segment, despite the circumstance that it bears four pairs of appendages, the more careful, philosophical observers have recognized the fact that it is composed of more than a single segment. Burmeister recognized only two segments in the head; Carus and Audouin recognized three; Macleay and Newman four; Straus-Durckheim even so many as seven. Huxley supposed that there are five segments bearing appendages, remarking, “if the eyes be taken to represent the appendages of another somite, the insect head will contain six somites.” (Manual of Anat. Invert. Animals, p. 398.)

These discordant views were based on the examination of the head in adult insects; but if we confine ourselves to the imago alone, it is impossible to arrive at a solution of the problem.

Newport took a step in the right direction when he wrote: “It is only by comparing the distinctly indicated parts of the head in the perfect insect with similar ones in the larva that we can hope to ascertain the exact number of segments of which it is composed.” He then states that in the head of _Hydroüs piceus_ are the remains of four segments, though still in the next paragraph, when speaking of the head as a whole, he considers it as the first segment, “while,” he adds, “the aggregation of segments of which it is composed we shall designate individually _subsegments_.”

That the head of insects is composed of four segments was shown on embryological grounds by the writer (1871) and afterwards by Graber (1879). The antennæ and mouth-parts are outgrowths budding out from the four primitive segments of the head; the antennæ grow out from the under side of the procephalic lobes, and these should therefore receive the name of antennal lobes. In like manner the mandibles and first and second maxillæ arise respectively from the three succeeding segments.

FIG. 34.—Embryo of _Anurida maritima_: _tc. ap_, minute temporary
appendage of the tritocerebral segment, the premandibular appendage;
_at_, antenna; _md_, mandible; _mx_^1, first maxilla; _mx_^2, second
maxilla; _p_^1–_p_^3, thoracic; _ap_^1, _ap_^2, abdominal
appendages; _an_, anus—After Wheeler.
]

While the postoral segments and their appendages are readily seen to be four in number, the question arises as to whether the eyes represent the appendages of one or more preoral segments. In this case embryology thus far has not afforded clear, indubitable evidence. We are therefore obliged to rely on the number of neuromeres, or primitive ganglia. In the postoral region of the head, as also in the trunk, a pair of neuromeres correspond to each segment. (See also under Nervous System, and under Embryology.) We therefore turn to the primitive number of neuromeres constituting the procephalic lobes or brain.

From the researches of Patten, Viallanes, and of Wheeler, especially of Viallanes, it appears that the brain or supraœsophageal ganglion is divided into three primitive segments. (See Nervous System, Brain.) The antennæ are innervated from the middle division or deutocerebrum. Hence the ocular segment, _i.e._ that bearing the compound and simple eyes, is supposed to represent the first segment of the head. This, however, does not involve the conclusion that the eyes are the homologues of the limbs, however it may be in the Crustacea.

The second head-segment is the antennal, the antennæ being the first pair of true jointed appendages.

The third segment of the head is very obscurely indicated, and the facts in proof of its existence are scanty and need farther elucidation.