14. The phthisogyne arises from a female larva under the same conditions as the phthisaner, and differs from the typical female in the same characters, namely absence of wings, stenonoty, microcephaly and microphthalmy. It is unable to attain to the imaginal instar.

15. The worker (ergates) is characterized by the complete absence of wings and a very small (stenonotal)

thorax, much simplified in the structure of its sclerites (Fig. 8, C, etc.). The eyes are small and the ocelli are usually absent or, when present, extremely small. The gaster is small, owing to the undeveloped condition of the ovaries. A receptaculum seminis is usually lacking, and the number of the ovarian tubules is greatly diminished. The antennæ, legs and mandibles are well developed.

FIG. 58. Pseudogyne of Myrmica sulcinodoides, with vestige of fore wing on left side. (Original.)

16. The gynæcoid is an egg-laying worker. It is a physiological rather than a morphological phase, since it is probable that all the worker ants when abundantly fed become able to lay eggs. Wasmann (1904a) observed in colonies of Formica rufibarbis that one or a few workers became gynææcoid and functioned as substitution queens. genus Leptogenys (including the subgenus Lobopelta (Fig. 137), and probably also in Diacamma and Champsomyrmex), the queen phase has disappeared and has been replaced by the gynæcoid worker.

In colonies of the Ponerine

17. The dichthadügyne, or dichthadii form female is peculiar to the ants of the subfamily Doryline and probably represent a further development of the gynæcoid. It is wingless and stenonotal, destitute of eyes or ocelli, or with these organs very feebly developed, and with a huge elongated gaster and extraordinary, voluminous ovaries (Figs. 141, A, and 147, b and c).

18. The macrergate is an unusually large worker form which in some species is produced only in populous or affluent colonies (Formica, Lasius).

19. The micrergate, or dwarf worker, is a worker of unusually small stature. It appears as a normal or constant form in the first brood of all colonies that are founded by isolated females.

20. The dinergate, or soldier (Figs. 52, A ; 60, a), is characterized by

a huge head and mandibles, often adapted to particular functions (fighting and guarding the nest, crushing seeds or hard parts of insects), and a thoracic structure sometimes approaching that of the female in size or in the development of its sclerites (Pheidole).

21. The desmergate is a form intermediate between the typical worker and dinergate, such as we find in more or less isolated genera

FIG. 59. Aphanogaster picea, an ant with monomorphic workers. (Photograph by J. G. Hubbard and Dr. O. S. Strong.)

of all the subfamilies except the Ponerinæ, e. g., in Camponotus (Fig. 45, a), some species of Pheidole (Fig. 52, b-e), Solenopsis and Pogonomyrmex, Azteca, Dorylus (Fig. 62, b), Eciton, etc. The term might also be employed to designate the intermediate forms between the small and large workers in such genera as Monomorium, Formica, etc.

22. The plerergate, "replete," or "rotund," is a worker, which in its callow stage has acquired the peculiar habit of distending the gaster with stored liquid food ("honey ") till it becomes a large spherical sac and locomotion is rendered difficult or even impossible (Figs. 218 and 219). This occurs in the honey ants (some North American species of Myrmecocystus, some Australian Melophorus and Camponotus, and to a less striking extent in certain species of Prenolepis and Plagiolepis). 23. The pterergate is a

worker or soldier with vestiges of wings on a thorax of the typical ergate or dinergate form, such as occurs in certain species of Myrmica and Cryptocerus (Fig. 63).

24. The mermithergate is an enlarged worker, produced by Mermis parasitism and often presenting dinergate characters in the thorax and minute ocelli in the head (Fig. 254, B, C). 25. The phthisergate, which corresponds to the phthisogyne and phthisaner, is a pupal worker, which in its late larval or semipupal stage has been attacked and partially exhausted of its juices by an Orasema larva (Fig. 252, B and C). It is characterized by extreme

b

stenonoty, microcephaly and microphthalmy, and is unable to pass on to the imaginal stage. It is in reality an infra-ergatoid form.

26. The gynandromorph is an anomalous individual in which male and female characters are combined in a blended or more often in a mosaic manner (Figs. 64 and 65).

27. The ergatandromorph (Fig. 66) is an anomaly similar to the last but having worker instead of female characters combined with those of the male (Wheeler. 1903b).

It is usually conceded that the fertilization or non-fertilization of the egg of the social Hymenopteron determines whether it shall give

rise to a male or a female. And as the queen represents the typical female form of the species, the problem of polymorphism is to account for the various worker forms, and those like the soldiers, pseudogynes and ergatoid females which are intermediate between the worker and the queen. The ergatomorphic males are usually regarded as inheriting worker characters. Thus the problem of polymorphism centers in the development. of the worker. It must suffice in this place to give the briefest possible statement of the views of the various authors who have endeavored to account for the development of this caste. These authors may be divided into three groups:

I. Those who believe with Weismann that the various castes are represented in the egg by corresponding units (determinants). Fertilization is then regarded as the stimulus which calls the female determinants into activity and meagre feeding the stimulus which arouses the workerproducing determinants in young larvæ arising from fertilized eggs. Such an explanation is obviously little more than a restatement or "photograph" of the FIG. 61. Workers major problem. It seeks to account for the and minor of Camponotus ameri- adaptive characters of the worker forms canus. (Photograph by J. G. Hubbard and Dr. O. S. Strong.) by natural selection acting on fortuitous

congenital variations.

2. Those who believe with Herbert Spencer that there is no such predetermination of the various female castes, but that these are produced epigenetically by differences in the feeding of the larvæ. The workers simply arise from larvæ that are inadequately fed but are nevertheless able to pupate and hatch when only a part of their growth has been completed. This is not, like the preceding view, a restatement of the problem, since the modifications produced by inadequate feeding are conceived as somatic and not as germinal, but it fails to explain how the worker caste acquires its adaptive characters, unless this caste is supposed to reproduce with sufficient frequency to transmit acquired somatic modifications to the germ-plasm of the species.

3. A third group of investigators believes with Emery that the germ-plasm of the social Hymenopteron is indeed implicated in the problem, not as possessing separate sets of determinants, but as being

d.

a

in a labile or sensitive condition and therefore capable of being deflected by differences in the trophic stimuli acting on the larva. According to Emery: "The peculiarities in which the workers differ from the corresponding sexual forms are, therefore, not innate or blastogenic, but acquired, that is somatogenic. Nor are they transmitted as such, but in the form of a peculiarity of the germ-plasm that enables this substance to take different developmental paths during the ontogeny. Such a peculiarity of the germ may be compared with the hereditary predisposition to certain diseases, which like hereditary myopia, develop only under certain conditions. The eye of the congenitally myopic individual is blastogenetically predisposed to short-sightedness, but only becomes shortsighted when the accommodation apparatus of the eye has been overtaxed by continual exertion. Myopia arises, like the peculiarities of the worker ants, as a somatic affection on a blastogenic foundation.

FIG. 62. Heads of workers of Dorylus affinis drawn to same scale to show differences in size and in number of antennal joints. (Emery.) a, Soldier, or worker maxima, 11 mm. long; b, worker major 5 mm. long; c, worker minima with 11-jointed antennæ; d, worker minima with 10-jointed antennæ; e, with 9-jointed antennæ, f, with 8jointed antennæ ; f', antenna of same enlarged.

"With this assumption the problem of the development of workers seems to me to become more intelligible and to be brought a step nearer its solution. The peculiarities of the Hymenopteran workers are laid down in every female egg; those of the termite workers in every egg of either sex, but they can only manifest themselves in the presence of specific vital conditions. In the phylogeny of the various species of ants the worker peculiarities are not transmitted but merely the faculty of all fertilized eggs to be reared as a single or several kinds of workers. The peculiar instinct of rearing workers is also transmitted, since it must be exercised by the fertile females in establishing their colonies."

The views above cited show very clearly that authors have