The development of what we may look on as the normal form of Termitidae—that is, the winged Insects male and female—is on the whole similar to that we have sketched in Orthoptera; the development in earwigs being perhaps the most similar. The individuals of Termitidae are, however, in the majority of cases if not in all, born without eyes; the wing-rudiments develop from the thoracic terga as shorter or longer lobes according to the degree of maturity; as in the earwigs the number of joints in the antennae increases as development advances. All the young are, when hatched, alike, the differences of caste appearing in the course of the subsequent development; the most important of these differences are those that result in the production of two special classes—only met with in social Insects—viz. worker and soldier. Of these the workers are individuals whose development is arrested, the sexual organs not going on to their full development, while other organs, such as the eyes, also remain undeveloped; the alimentary canal and its adjuncts occupy nearly the whole of the abdominal cavity. The adult worker greatly resembles—except in size—the young. Grassi considers that the worker is not a case of simple arrest of development, but that some deviation accompanies the arrest.
The soldier also suffers an arrest of development in certain respects similar to the worker; but the soldier differs in the important fact that the arrest of the development of certain parts is correlative with an extraordinary development of the head, which ultimately differs greatly from those of either the worker or of the sexual males and females.
Fig. 233.—The pairs of mandibles of different adult individuals of Termes sp. from Singapore. A, Of worker; B, of soldier; C, of winged male; D, of winged female.
Soldier.—All the parts of the head of the soldier undergo a greater or less change of form; even the pieces at its base, which connect it by means of the cervical sclerites with the prothorax, are altered. The parts that undergo the greatest modification are the mandibles (Fig. 233, B); these become much enlarged in size and so much changed in form that in a great many species no resemblance to the original shape of these organs can be traced. It is a curious fact that the specific characters are better expressed in these superinduced modifications than they are in any other part of the organisation (except, perhaps, the wings). The soldiers are not alike in any two species of Termitidae so far as we know, and it seems impossible to ascribe the differences that exist between the soldiers of different species of Termitidae to special adaption for the work they have to perform. Such a suggestion is justifiable only in the case of the Nasuti (Fig. 234, 1), where the front of the head is prolonged into a point: a duct opens at the extremity of this point, from which is exuded a fluid that serves as a cement for constructing the nest, and is perhaps also used to disable enemies. Hence the prolongation and form of the head of these Nasuti may be fairly described as adaptation to useful ends. As regards the great variety exhibited by other soldiers—and their variety is much greater than it is in the Nasuti—it seems at present impossible to treat it as being cases of special adaptations for useful purposes. On the whole it would be more correct to say that the soldiers are very dissimilar in spite of their having to perform similar work, than to state that they are dissimilar in conformity with the different tasks they carry on.
Fig. 234.—Soldiers of different species of Termites. (After Hagen.) 1, Termes armiger; 2, T. dirus; 3, Calotermes flavicollis; 4, T. bellicosus; 5, T. occidentis; 6, T. cingulatus (?); 7, Hodotermes quadricollis (?); 8, T. debilis (?), Brazil.
The Termite soldier is a phenomenon to which it is difficult to find a parallel among Insects. The soldier in the true ants is usually not definitely distinguished from the worker, but it is possible that in the leaf-cutting ants, the so-called soldier may prove to be more similar in its nature to the Termite soldier. The soldiers of any one species of Termite are apparently extremely similar to one another, and there are no intermediates between them and the other forms, except in the stages of differentiation. But we must recollect that but little is yet known of the full history of any Termite community, and it is possible that soldiers which in the stage of differentiation promise to be unsatisfactory may be killed and eaten,—indeed there is some evidence to this effect. There is too in certain cases some difference—larger or smaller size being the most important—between the soldiers of one species, which may possibly be due to the different stage of development at which their differentiation commenced.
It would at present appear that, notwithstanding the remarkable difference in structure of the soldiers and workers of the white ants, there is not a corresponding difference of instinct. It is true that soldiers do more of certain things than workers do, and less of others, but this appears to be due solely to their possession of such very different structures; and we are repeatedly informed by Grassi that all the individuals in a community take part, so far as they are able, in any work that is going on, and we find also in the works of other writers accounts of soldiers performing acts that one would not have expected from them. The soldiers are not such effective combatants as the workers are. Dudley and Beaumont indeed describe the soldiers as merely looking on while the workers fight.[[295]] So that we are entitled to conclude that the actions of the soldiers, in so far as they differ from those of the rest of the community, do so because of the different organisation and structures of these individuals. We shall, when speaking of food, point out that the condition of the soldier in relation to food and hunger is probably different from that of the other forms.
Various Forms of a Community.—The soldiers and workers are not the only anomalous forms found in Termitid communities; indeed on examining a large nest a variety of forms may be found that is almost bewildering. Tables have been drawn up by Grassi and others showing that as many as fifteen kinds may be found, and most of them may under certain circumstances coexist. Such tables do not represent the results of actual examination in any one case, and they by no means adequately represent the number that, according to the most recent observations of Grassi, may be present; but we give one taken from Grassi, as it conveys some idea of the numerous forms that exist in certain communities. In this table the arrangement, according to A, B, C, D, E, represents successive stages of the development:—