certain species (Lasius niger, Atta) may develop into workers. The Dzierzon theory cannot, therefore, be rigorously extended to the ants till this matter has been more thoroughly investigated.

No complete embryological study of the ants has yet been undertaken. The earliest account of the development of these insects is by Ganin (1869). He studied the eggs of Lasius flavus, Formica fusca, Myrmica levinodis and ruginodis and Tetramorium cespitum, and described the formation of the amnion. This envelope, he maintained, arises by delamination from the blastoderm, but as his investigations. were made before modern embryological methods were introduced, it

is impossible to attach much importance. to his statements. Several years ago Blochmann published two short papers (1884, 1886) on the growth of the ovarian egg, and the formation of the polar bodies. and blastoderm in Camponotus ligniperdus and Formica fusca. I have examined some of the later stages of F. gnava and find them to be very similar to those of the bee (Apis, Chalicodoma) and wasp (Polistes). The accompanying sketch (Fig. 35) of a young embryo of F. gnava is interesting as showing some of the conservative traits in the development of ants. Not only are there distinct traces of the antennæ (not seen in the figure, as the head is folded over the anterior pole of the egg) and three pairs of thoracic legs, but there are also traces of the abdominal appendages, although all of these disappear before the hatching of the larva. The thoracic limbs and antennæ again develop in the larval stage, but the evanescent abdominal appendages, with the exception of those that go to form the parts of the sting of the adult, are mere vestiges, harking back, so to speak, to the legs of ancient larval types like the caterpillar, or eruciform larva of the sawflies or even to the Palæodictyopteroid ancestors of all insects.

FIG. 38. Larva of Stigmatomma pallipes. (Original.) a, Larva from the side; b, flexuous hair enlarged; c, head from above.

The larva emerges from the egg as a soft, legless, translucent grub, usually shaped like a "crook-necked" squash or gourd, with a broad, straight posterior and narrower, curved anterior end terminating in a small but distinct head (Fig. 36). In some forms (Eciton, Parasyscia, Lobopelta, etc.) the body is more cylindrical (Fig. 37). In all cases, however, it consists of a head and thirteen more or less clearly marked segments. Three of these belong to the thorax, the remainder to the

abdomen, i. e., to the pedicel plus the gaster of the adult. In a few genera (Pseudomyrma) the head, as Emery (1899e) has shown, bears minute vestiges of antennæ. The mouth-parts are distinct and consist of a pair of mandibles, a pair of fleshy maxillæ and an unpaired labium. Both maxillæ and labium are furnished with small, conical tactile papillæ (Figs. 38-41), and the latter also bears the opening of the sericteries, or spinning glands. No traces of eyes are visible. There are ten pairs of tracheal openings, a pair each for the meso- and metathoracic and the eight anterior abdominal segments.

The transparent chitinous integument is very thin and easily ruptured, so that handling the larvæ must require considerable care on the part of the ants. It is sometimes naked (Platythyrea), but much more frequently covered with chitinous hairs which in different species show a bewildering diversity of form and are most abundant and conspicuous in young individuals. These hairs, which Janet has called "poils d'accrochage," may be long, simple and rigid, or flexuous, helicoid, furcate or tipped with single or double hooks, ramose, plumose or serrate (Figs. 3743). Some species have hairs of very different kinds on different parts of the body. These are all adaptive structures with well-defined functions, at least in certain species. The following are some of these functions:

*

FIG. 39. Larva of Lobopelta elongata. (Original.) a, Young; b, adult larva; c, head of adult larva from above; d, tubercle of young, e, tubercle of adult larva.

1. The hairs may serve to protect the delicate larvæ from the mandibles of their voracious or underfed sister larvæ.

2. They prevent the body of the larva from lying directly in contact with the moist soil of the nest.

3. In many Myrmicinæ the pairs of very long, hooked, dorsal hairs which have an S- or C-shaped flexure in their bases (Fig. 43), serve to anchor the larvæ to the walls of the nest, the under surfaces of stones, etc. Janet (1904, p. 32) has shown that the flexure acts like a spring and prevents the rupture of the thin integument when the larva is hastily picked up by the ants, for when the hair is drawn out, the terminal hooks have time to become inclined and release their hold.

4. The hairs hold the young larvæ together in packets and thus

function like the salivary coating of the eggs, in enabling the ants to transport large numbers of their offspring with little effort. This is a matter of great moment when the colony is disturbed or attacked and the young have to be carried away and concealed with great dispatch.

FIG. 40. Eggs and larvæ of Pachycondyla harpax. (Original.) a, Eggs; b, young larva with pointed tubercles; c, tubercle of same enlarged; d, adult larva with boss-like tubercles; e, head of same from above.

Besides hairs, the larvæ of many Ponerine genera (Lobopelta, Pachycondyla, Ponera, Diacamma, etc.) have prominent, pointed, or rounded tubercles which probably have a protective function (Figs. 39-41), and in addition to these, Ponera has pairs of glutinous dorsal tubercles (Fig. 41) which, like the flexuous, hooked hairs of many Myrmicinæ, serve to attach the larva to the walls of the nest (Wheeler, 1900e).

The feeding of the larvæ is of considerable interest owing to the prevailing supposition that the quantity or quality of the food, or both, determine whether the larva hatching from a fertilized egg shall become a worker or a female. Recent observations have shown that the different species adopt very different methods of nourishing their brood. Many ants, like most Camponotinæ, Dolichoderinæ and Myrmicinæ, feed their larvæ only on regurgitated liquids, whereas the Ponerinæ, many Myrmicinæ and probably also the Dorylinæ, feed them directly with pieces of the same kind of food that they bring into the nest for their own consumption. Carnivorous species give their larvæ pieces of insects, and the harvesting ants (Pogonomyrmex, some species of Pheidole) administer fragments of seeds. In larvæ that are habitually fed on such resistant substance the mandibles are apt to be more highly developed. Some species (Aphænogaster, Lasius, Pheidole, etc.) undoubtedly feed their brood both with regurgitated and solid food. Certain groups of ants that have developed a specialized diet in their adult stages show a corresponding specialization in feeding their young. Thus the larvæ of the fungus-growing Attii of tropical America are nourished with wisps of fungus-hyphæ, and according to Dahl (1901, p. 31) the larvæ of the East Indian Campo

notus quadriceps feed directly on the pith of the plants in which the insects nest (see Fig. 168).

The internal structure of the larva, which is essentially that of the mature embryo, has been described by Ganin (1876), Dewitz (1877, 1878), Nassonow (1886) and Karawaiew (1898, 1900) for Formica, Myrmica and Lasius, by Berlese (1901) for Tapinoma, and by Pérez (1902) for Formica rufa. In the following account I have followed Pérez. The alimentary tract of the larva (Fig. 44) is much simpler than that of the adult described in Chapter III. The thin outer integument is folded into the body to form the walls of a slender pharynx and œsophagus, which is, therefore, of ectodermal origin and corresponds to the stomodæum of the embryo. Muscles extend from the walls of the pharynx to the dorsal integument and function as dilators during the sucking or imbibing movements of the larva. At its posterior end the pharynx opens on an elevated, valvular papilla into the chylific stomach, which is equivalent to the embryonic mesenteron, or mid-gut. It is a spacious, ovoid receptacle, somewhat attenuated anteriorly in the future. proventricular region and closed posteriorly where it unites with the anterior end of the hind-gut, or embryonic proctodæum. The latter formed by a tubular invagination of the ectoderm similar to that which forms the œsophagus. Owing to the closure between the chylific stomach and the hind-gut, all the undigested portions of the larval food enclosed in the series of successively sloughed peritrophic membranes, accumulate in the cavity of the stomach and form a black, elliptical mass, the meconium, which often shows through the translucent body walls of the larva. The hind-gut is differentiated into two regions, a more slender, tubular anterior portion, the small intestine, and a more capacious sac, the large intestine. The latter is constricted behind and opens on the surface as the anus, which has the form of a transverse slit and is provided with a sphincter muscle. Four Malpighian tubules open into the anterior blind end of the small intestine and describe a few convolutions in the dorsal body cavity. In the ventral body cavity lies a pair of less

is

FIG. 41. Larva of Ponera pennsylvanica. (Original.) a, Larva ready to pupate; b, bristle-capped tubercle of same; c, head from above.

convoluted sericteries, or spinning glands, which unite to form a duct, opening on the tip of the labium. These organs are also present in larvæ that do not spin cocoons. The nervous system consists of a cerebral ganglion, connected by a pair of commissures surrounding the œsophagus, with the most anterior of a series of twelve ganglia, which extend through the body on the ventral side of the alimentary tract. The first, or subœsophageal ganglion, is a fusion of the ganglia of the mandibular, maxillary and labial segments of the embryo. The tubular heart lies just under the dorsal integument and terminates anteriorly beneath the brain. The pericardial cells float out into the body like a velum on each side of the heart. The feebly developed muscular system consists of longitudinal fibres lying in two latero-dorsal and two lateroventral zones in the various segments. Another set of muscles, which are oblique antero-posteriorly, have their dorsal insertions at the intersegmental constrictions near the stigmata and their ventral insertions at the anterior border of the preceding segment. Segmental groups of

FIG. 42. Larvæ of Pogonomyrmex molefaciens. (Original.) a, Adult larva; b, young larva; c, hair of same enlarged.

nocytes are suspended near these oblique muscles. The tracheal system consists of a pair of longitudinal trunks which send off short branches to the adjacent stigmata. The greater portion

of the spaces between the above-described organs is filled with lobes of the voluminous fat-body which shows through the transparent integument and gives the larva its shining white, greenish or pinkish color. The undeveloped reproductive organs are clearly discernible as small bodies in the postero-dorsal region of the abdomen, and the histoblasts, or imaginal discs, are present as small, paired clusters of formative cells in the hypodermis of the integument. They represent the adult antennæ, legs, wings, copulatory organs, parts of the sting, etc. Each histoblast receives a slender nerve.

When the larva approaches its full size important changes occur in both its external and internal structure in preparation for metamorphosis, or pupation. For an account of the internal changes, which are too numerous and intricate to be described here, the reader is referred to the works of Karawaiew, Berlese and Pérez. The external changes may be briefly considered. When full-grown the larva passes