Fig. 352.—Pupation of Proctotrypes sp. in body of a beetle larva.
Comparatively little is known as to the habits of the members of this family, but such information as has been obtained leads to the conclusion that great variety will be found to exist in this respect. We have already mentioned that numerous species have been ascertained to feed inside the eggs of Insects or of Spiders; others have been reared from larvae or from galls of the minute Dipterous midges of the family Cecidomyiidae; others have been obtained from Cynipid galls, a few from ants' nests and from green-fly; some species are known to attack Coleoptera. The distinguished Irish entomologist, Haliday, has written an account of the proceedings of a species of Bethylus,[[440]] from which it has been supposed that this Insect carries off living caterpillars, and stores them in a suitable receptacle as food for its progeny, thus anticipating, as it were, the habits of the fossorial division of the Aculeata, in which group this instinct has, as we shall subsequently relate, attained an astonishing degree of perfection. Haliday's observation was unfortunately incomplete and has not been subsequently confirmed. The Bethylides are remarkable for their great approach in structure to the Aculeates, so much so that entomologists are not agreed as to whether certain Insects are Proctotrypids or Aculeates. Pristocera, with a very wide distribution, may be mentioned as illustrative of these doubtful forms; but other genera of the Bethylides are in many respects very similar to the Aculeates, and it is not matter for surprise that Haliday should have considered the Bethylides to be a tribe of the stinging Hymenoptera. The genus Scleroderma consists of small Insects much resembling ants, and, as well as some of its allies, is of great interest from the remarkable phenomena of polymorphism presented by certain species. The males in this genus are winged, the females completely apterous; yet at times winged females are produced—as exceptional individuals in a brood of wingless specimens—the females in these cases being not only winged, but possessed of ocelli like the females of other winged Hymenoptera. Particulars of a case of this kind have been given by Sir Sidney Saunders,[[441]] and Ashmead also mentions[[442]] the exceptional occurrence of these winged females. Westwood[[443]] was of opinion that there are three forms of the female sex. This subject is of importance in connexion with the production of the various castes in ants. Although the presence of wings in these Insects is always accompanied by the existence of ocelli (which, it will be remembered, are normally absent from the wingless individuals), yet the converse is not always the case, for a form of the female of Cephalonomia formiciformis, without any wings, yet having ocelli, as well as eyes, well developed, is figured by Westwood.[[444]]
Fig. 353.—Cyclops form of larva of Platygaster sp. (After Ganin.) a, Mouth; b, antenna; c, claw-limb; d, lower lip (the pointing line is a little too short); e, doubtful "zapfenförmig" organ; f, wing-like lobe; g, branch of the tail.
The development of some of the Proctotrypids has been partially described by Ganin and others, and is of an extraordinary character. Ganin's observations[[445]] were most complete in the case of a species of Platygaster, which he found in the larva of a very minute Dipteron of the genus Cecidomyia. The Platygaster larva changes its form very much in the course of its life, resembling at first a minute Crustacean rather than an Insect-larva; it has a very large rounded anterior portion, while behind it terminates in two, or more, tail-like processes. By a very peculiar kind of metamorphosis this Cyclops-like larva changes into an almost unsegmented, oviform larva, destitute of appendages; by a second change this creature assumes a third condition, in which it is similar to the ordinary form of parasitic Hymenopterous larvae. Sometimes several of the Platygaster larvae are found in a single host, but only one of them reaches this third stage. Afterwards the third larval instar passes into the pupal stage, which lasts five or six days, and then the perfect Insect appears. It is worthy of remark that the internal organs undergo quite as remarkable a change as the outer form does. The metamorphoses of some other Proctotrypidae have been examined by Ganin, and appear to be of an equally interesting character.[[446]]
There is reason to suppose that these Platygaster parasites are of great economic importance as well as of scientific interest, for Platygaster herrickii is one of the enemies of the larva of the destructive Hessian fly, Cecidomyia destructor.
The Proctotrypidae are no doubt extremely numerous in species, but as yet they have been very little studied; a good work on the British species is much required. A valuable contribution has recently been made to the study of the family by Ashmead, in the book we have already referred to. This volume includes much information on the natural history of these Insects, and the outline figures give some idea of the great variety of external form.
Fig. 354.—Alaptus excisus, Westwood. Britain. (Probable size about ½ millim.)