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

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

FIG. 60. Pheidole borinquenensis of Porto Rico. (Original.) a, Soldier; b, same in profile; c, worker.

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. 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

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.

"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

been impressed by very different aspects of the complicated phenomena of polymorphism, and that each has emphasized the aspect which seemed the most promising from the standpoint of the general evolutionary theory he happened to be defending. Escherich (1906) has recently called attention to two very different ways of envisaging the problem; one of these is physiological and ontogenic, the other ethological and phylogenetic. As these furnish convenient captions under which to continue the discussion of the subject, I shall adopt them, and conclude with a third, the psychological aspect, which is certainly of sufficient importance to deserve consideration.

While the ontogeny of nearly all animals is a repetition or reproduction of the parent, this is usually not the case in the social Hymenoptera, since the majority of the fertilized eggs do not give rise to

FIG. 63. Vestigial wings in worker ants.

(Original.) A, Myrmica scabrinodis var., with spatulate wing vestiges on mesothorax; B, and C, Thoraces of two other individuals from the same colony, showing a more vestigial development of the wings; D, Soldier of Cryptocerus aztecus with mesothoracic wing vestiges.

queens but to more or less aberrant organisms, the workers. And as these do not, as a rule, reproduce, the whole phenomenon is calculated to arouse the interest of both the physiologist and the embryologist. The former, concentrating his attention on the reactions of the animal to the stimuli proceeding from its environment, is inclined to study its later stages as determined by the reactions to such stimuli, without regard to any internal or hereditary predetermination or disposition, while the embryologist seeks out the earliest moment at which the organism may be shown to deviate from the ontogenetic pattern