Spencer's generalisation is not inconsistent with the facts hitherto brought to light, though it is possible that the progress of knowledge may show some variety as to the periods of the development at which the commencements of the modifications occur.
Fig. 339 represents the chief castes, or adult forms, existing in a community of one of the most highly developed of the species of social Hymenoptera, the leaf-cutting ant, Atta cephalotes. We shall, when dealing with Formicidae, enter into some details as to these and other cases of polymorphism.
Fig. 339.—Adult forms of Atta (Oecodoma) cephalotes, taken from a nest in Trinidad by Mr. J. H. Hart, 25th June 1895. A, male; B, winged female; C-F, various forms unwinged; C, so-called soldier; D, large worker; E, smaller worker; F, smallest worker or nurse. All equally magnified (one and half times).
Our object at present is to bring to the eye of the reader the great diversity of outer form that is believed, rightly or wrongly, to result from the mode of treatment of the young. And we will also take this opportunity of more fully illustrating the remark we made on p. [85] as to the profound distinctions that exist between ants and white ants, or Termites, notwithstanding the remarkable analogies that we shall find to exist in many of their social arrangements.
The analogies we allude to, coupled with the fact that there is a certain general resemblance in outer form between the workers of Termites and ants, and even between the extraordinary castes called soldiers in the two groups, have given rise to the idea that there is a zoological relationship between the social forms of Neuroptera and Hymenoptera. The two are, however, zoologically amongst the most different of Insects. The external skeleton in Termites is remarkable for its imperfect development, the sclerites being small and isolated, while the segmental differentiation of the body is low (Fig. 225, etc.), so that there is no difficulty in counting the segments. In ants the reverse is the case as regards both these facts, the various segments being most unequal, so that their homologies have only been detected after prolonged studies, while the chitinisation and articulation of the various parts is so complete that the ant may be described as cased in armour, fitting together so exactly that it is difficult anywhere to introduce the point of a needle into its chinks. The wings of the two kinds of Insects are also extremely different. The differences between the modes of growth and development of the two sets of Insects are as profound as the distinctions in their anatomy. Termitidae belong to the division of Insects in which the wings are developed outside the body; Hymenoptera to the division in which they are developed inside the body. In Termites the growth of the individual is slow, and the final form is reached gradually. In the ants the growth is carried on with great rapidity, and during it the Insect is a helpless maggot absolutely dependent on the attentions of its seniors, while the difference in form and structure between the ant-larva and the ant are enormous. Both anatomy and ontogeny are profoundly different in ants and Termites. To these distinctions must be added, as of much importance, the fact that in Hymenoptera only the female sex is modified for the division of labour, while in Termites both sexes undergo this change. Hence it is impossible to suppose that the remarkable analogies that exist between the societies of ants and those of Termites are due to any common origin. It is probably to some similar physiological susceptibilities in the ancestors, at an extremely remote epoch, of both groups that we must look for an explanation of the interesting resemblances in the social lives of ants and Termites.
The Hymenoptera are no doubt one of the largest Orders of Insects, the species of the parasitic tribes being apparently innumerable. No doubt 250,000 species of the Order exist, and possibly the number may prove to be very much larger. Up to the present time 25,000 or 30,000 have been discovered. No remains of Insects of this Order, of older age than the Lias, have been brought to light; it is indeed doubtful whether the fossils considered to be Hymenopterous of the period referred to are really such.
The Order, as already mentioned, consists of two very distinct sub-Orders, viz.:—
1. Hymenoptera Sessiliventres.—Insects with the abdomen broad at the base, its first segment not completely amalgamated with the thorax.
2. Hymenoptera Petioliventres or Petiolata.—The abdomen connected with what appears to be the thorax by a slender joint, the posterior part of the apparent thorax consisting of an abdominal segment.