the mesothoracic ganglion arises a pair of so-called alar nerves, which innervate the great longitudinal and transverse vibratory muscles of the wings. The musculature of the epinotum, petiole and postpetiole is supplied by the first to third abdominal ganglia, the two first of which are fused with the metathoracic ganglion. The fourth abdominal (first gastric in the Myrmicida) remains in the segment to which it belongs, but lies at its extreme anterior edge. As both this and the succeeding. gastric ganglia have been secondarily drawn forward, the pairs of nerves which they give off run obliquely backward, to their innervations. Janet (1902) has found that each of the two nerves arising from each of the four anterior gastric ganglia divides into a dorsal and a ventral trunk. The former sends off a sensory nerve to the corresponding dorsal quadrant of the segment and three motor nerves to its three muscles, the latter a sensory nerve to the ventral quadrant and six motor nerves to as many muscles. The sensory nerves go to the sense-hairs of the integument. The terminal (fifth gastric) ganglion, formed, as we have seen, by a fusion of the eighth to tenth abdominal ganglia, sends off four pairs of nerves, the first to the sense organs and muscles of the stylets of the sting, the second to the sense organs and muscles of the gorgeret, the third to the anal sphincter and papilla, and the fourth to the walls of the hind gut.

The Sympathetic.-This consists of several minute ganglia and nerves connected with the central nervous system and supplying the musculature of the alimentary tract. It is, to judge from Janet's account of Myrmica (1902), well developed in ants and not unlike that of other insects. It may be said to embrace two systems, one supraintestinal and supplying the dorsal and lateral portions of the digestive tract, the other subintestinal and lying beneath the intestine. and above the ventral nerve-cord. The supraintestinal system may be divided into an unpaired and a paired portion. The former begins in the small frontal ganglion (Fig. 27, fgl), which lies anterior to the brain, to which it is joined by a pair of connectives. According to Janet, these connectives arise in the protocerebrum, but other authors believe that they are of tritocerebral origin. The frontal ganglion sends a pair of coalesced nerves to the supero-anterior wall of the pharynx and a much stouter unpaired nerve, known as the recurrens (ren), downward and backward along the dorsal wall of the pharynx, to a ganglion (the hypocerebral, hes), which lies on the oesophagus just beneath the protocerebrum. Besides innervating the oesophagus this ganglion sends back a pair of long, slender connectives (sym) along the sides of the cesophagus and crop to the point where the latter contracts to form the gizzard. Here each connective termi

nates in a so-called pre-stomachal ganglion, which innervates the surrounding wall of the crop and gizzard. The paired supraintestinal sympathetic has an anterior and a posterior portion. The former consists of the œsophageal ganglia, which lie on each side of the hypocerebral ganglion. They are united with this by commissures and with the tritocerebrum by connectives, and innervate the sides of the œsophagus and crop. The posterior portion of the paired system is very imperfectly known. Janet maintains that the fourth pair of nerves from the terminal ganglion of the ventral cord turns forward and innervates the posterior portion of the digestive tract in somewhat the same manner as the anterior portion is innervated by the brain through the frontal and œsophageal ganglia. He, therefore, calls these the proctodeal recurrent nerves. The subintestinal sympathetic system of Myrmica comprises a series of minute, unpaired, metameric ganglia connected with several of the ganglia of the ventral cord. This system, too, both in ants and in other insects, is imperfectly known.

The Sense-Organs.-The sense-organs of ants, like those of insects in general, are modifications of the integument and the terminations of sensory nerves. Hence there can be no sense-organs in the interior of the body unless they have been carried in secondarily on infoldings of the integument. As there are no openings anywhere in the chitinous investment of the insect's body, except those at the anterior and posterior ends of the midgut, the nerve terminations are never freely exposed on the surface, but always covered with at least a very delicate layer of chitin. The number and diversity of sense-organs in insects is very great, but nevertheless, attempts have been made to trace them all back to a common primitive type. One of the most recent of these attempts is that of Berlese (1907) who finds that nearly all these organs admit of hypothetical derivation from a "protesthesis," a sensilla, or sense-bud, consisting of one or a few chitin-secreting hypodermal cells, a gland cell and a nerve cell. It is possible to show that this type of structure keeps recurring in the various sense-organs of even such highly-specialized insects as the ants.

Tactile (Trichodeal) Sensillæ.-As stated in a previous chapter, ants are usually covered with hairs, which are coarse and long on the body and shorter and denser on the legs and especially on the antennæ. As all of these hairs are movably articulated to the general chitinous integument and are provided with fine nerve terminations, they are universally regarded as tactile sensillæ, although they also aid in the removal of the larval or pupal skin during ecdysis, for they are at first bent at their bases and applied to the chitinous layer to which they belong, but later, in becoming erect, loosen and push the overlying

exuvia away from the surface of the body. In section each hair is seen to be a hollow chitinous tube, closed at its apex and open at its base, which is bulbously swollen and fits into a ring-shaped thickening

FIG. 31. Trichodeal and campaniform sensillæ of ants. (Janet.) A, Trichodeal sensillæ from proximal border of fore coxa of female Lasius niger, X 1,000; B, single sensilla from the group represented in A, X 2,000; C, longitudinal section through tip of middle coxa, trochanter and base of femur of Myrmica rubra worker, X 100; D, cross-section of tip of hind tibia of M. rubra, X 200; E, F and G, sections of campaniform' sensillæ from tip of mandibles of M. rubra; H, campaniform sensillæ near articulation of wing of female Camponotus herculeanus, X 500; t, chitinous hair; c, chitinous integument; h, hypodermis; n, nerve-termination; u, bell, or umbrella, in the center of which the nerve terminates; x, groups of campaniform sense-organs; d, fossa or pit in the chitinous integument; p, pore in same.

of the chitinous integument (Fig. 31, A, B, Fig. 32, b). This tube is secreted by one or more large hypodermal cells, and a delicate nerve fiber extends up into its base. When the tip of the hair touches an

object the tactile impulse is evidently transmitted to the nerve through the movement of the bulbous base in its cup-shaped socket. There is, therefore, no essential difference between the tactile function of the hairs of ants and the analogous structure in mammals.

Olfactory and Gustatory Sensillæ.-It seems to be impossible to distinguish between these organs in insects, although it may be asserted that the organs of smell are situated mainly or exclusively on the antennæ, whereas, those of taste are found on the mouth-parts, especially on the maxillæ and labium and their palpi. The antennary sensillæ of ants have been studied by Hicks (1859), Leydig (1860), Forel (1874, 1884), Lubbock (1877), Kraepelin (1883), and more recently by Krause (1907). From the researches of these authors it appears that in addition to numerous tactile hairs like those described above, there are four more modified types of sensilla which have been more or less definitely connected with an olfactory function. These do

FIG. 32. Subdiagrammatic section of the antennal sense-organs of an ant. (Kraepelin.) a, Basiconic sensilla; b, trichodeal sensilla, or tactile hair; c, cœloconic sensilla; d, ampullaceous sensilla; f, flask-shaped sensilla; g and h, openings of same on surface of antenna; i, gland cells; k, chitinous integument.

not occur on the scape and first funicular joint of the antennæ, but only on the remaining joints and especially on the enlarged terminal joint, which possesses by far the greatest number of all the various sensillæ. The following is a very brief description of these extraordinary structures:

(a) Clubs of Forel (now called basiconic sensillæ by Berlese).These resemble the tactile hairs, but are conical and immovable at the base and their chitinous investment is exceedingly thin (Fig. 32, a). What corresponds to the cavity of the hair contains a dense bundle of delicate protoplasmic threads, which are prolongations from as many

large elliptical cells situated in the hypodermis. These cells form a compact mass, formerly supposed to be a ganglion, but now interpreted as a cluster of unicellular glands that secrete a liquid through the thin chitinous cap of the organ onto the surface of the antennæ. It is, indeed, difficult to conceive such sensillæ as having an olfactory function unless their exposed surfaces are moist like the olfactory organs in the mucous membranes of vertebrates. The nerve termination to the basiconic sensillæ applies itself to the cluster of gland cells and then breaks up into delicate branches that pass around and between the latter and up into the conical portion of the organ.

(b) Clubs Lying in Elliptical Pits (coloconic sensillæ of Berlese). -These may be derived from the preceding type by supposing that the conical hair has come to lie horizontally and to be enclosed in an elongated cavity in the chitinous integument (Fig. 32, c). The cellular structure of the organ is essentially the same as that of the basiconic sensillæ.

(c) Champagne-cork Organs of Forel (ampullaceous sensillæ of Berlese). These evidently represent a further modification of the cœloconic type, on the supposition that the hair becomes smaller and more erect and the pit in which it is enclosed becomes circular, much deeper and opens on the surface of the body by means of a small pore (Fig. 32, d).

(d) Flask-shaped Organs of Lubbock and Forel.-Hicks (1859) was the first to describe these extraordinary organs in Myrmica, but Forel and Kraepelin have given a more detailed account of their structure. They are really an extreme form of the ampullaceous sensilla, and may be derived from this by supposing that the chitinous ampulla has become enormously lengthened and attenuated till it forms a narrow sac enclosing the conical hair and connected with the pore in the integument by means of a slender tube running more or less parallel with the surface of the antenna (Fig. 32, g, h). That these sensillæ have developed from those of the preceding type (c) is shown by the existence. of transitional forms both in ants and in other Hymenoptera. The cellular portions of all these forms of ampullaceous sensillæ are essentially the same as those of types a and b.

The gustatory sensillæ, situated on the mouth-parts and including those in the terminal joints of the palpi, though resembling the antennary sensillæ, in general reproduce only the more primitive of the above-mentioned types, that is, those most like the typical tactile and basiconic sensilla. The more specialized ampullaceous types are found only in the antennæ.

The Chordotonal Organs.-Recent studies have shown that these