Riley has called attention[[458]] to some facts in connection with I. tritici and I. grande, that make it clear that these two supposed species are really alternate generations, and that both generations are probably in larger part, if not entirely, parthenogenetic. Some species of the genus Megastigmus are known to be of phytophagous habits.[[459]]
Fig. 358.—Blastophaga grossorum. A, Male, × 22; B, female, × 15. (After Mayer, Mitt. Stat. Neapel, iii. 1882.)
The most interesting of all the forms of Chalcididae are perhaps those called fig-Insects. A considerable number of species are now known, and amongst them we meet with the unusual phenomenon of species with wingless males, the females possessing the organs of flight normally developed. The wingless males exhibit the strangest forms, and bear no resemblance whatever to their more legitimately formed partners (Fig. 358, A, B). Many of the fig-Insects belong to a special group called Agaonides. Others belong to the group Torymides, which contains likewise many Chalcididae of an ordinary kind; possibly some of these may be parasitic on the Agaonides. Some of these Torymid fig-Insects have winged males, as is normal in the family, but in other cases winged and wingless forms of the male of one species may be present.
The most notorious of these fig-Insects is the one known as Blastophaga grossorum (Fig. 358), this being the chief agent in the custom known as caprification of the cultivated fig-tree. This process has been practised from time immemorial, and is at the present day still carried on in Italy and the Grecian archipelago. The Greek writers who describe it say that the wild fig-tree, though it does not ripen its own fruit, is absolutely essential for the perfection of the fruit of the cultivated fig. In accordance with this view, branches from the wild fig are still gathered at certain seasons and suspended amongst the branches of the cultivated fig-trees. The young fig is a very remarkable vegetable production, consisting of a hollow, fleshy receptacle, in which are placed the extremely numerous and minute flowers, the only admission to which is by a small orifice at the blunt end of the young fig; this orifice is lined with projecting scales, that more or less completely fill it up or close it; nevertheless inside this fruit the Blastophaga grossorum develops in large numbers. The males are, as we have seen, wingless creatures, and do not leave the fruit in which they were bred, but the females make their way out of the wild fig, and some of them, it is believed, enter the young fruit of the cultivated trees and lay their eggs, or attempt to do so, therein; and it has been supposed by various writers that these proceedings are essential to the satisfactory development of the edible fruit. It is a curious fact that the Blastophaga develops very freely in the wild fig—so much so, indeed, as to be a means of preventing it from coming to maturity; but yet the Insect cannot complete its development in the cultivated fruit. This is due to the fact that the fly must lay its egg in a particular part of the fig-ovule, so that when the egg hatches the larva may have a proper supply of food. In the cultivated fig the structure of the flower differs somewhat from that of the caprificus, as the wild fig is called, and so the egg, if deposited at all, does not reach a proper nidus for its development. Hence the Blastophaga can never live exclusively on the cultivated fig, and if it be really necessary for the development of the latter, must be brought thereto by means of the caprifig. Whether the Blastophaga be really of use, as has been for so long supposed, is, however, a matter for doubt. The reasons for this are (1) that those who think caprification beneficial do not agree as to the mode in which they suppose it to be so; (2) that there is but little reason for believing that when introduced amongst the cultivated figs the Blastophaga occupies itself to any great extent therewith; and (3) that in some parts of the world caprification is not performed, but the cultivated fig nevertheless ripens its fruit there. Hence many writers on the subject—Solms-Laubach,[[460]] Mayer,[[461]] and Saunders[[462]]—entertain considerable doubt as to whether caprification is at present anything more than an old custom destitute of practical utility. On the other hand, Riley states[[463]] "that the perfect Smyrna fig, the most esteemed of the edible species, can be produced only by the intervention of the Blastophaga psenes [grossorum]."
Although the questions connected with the effect the Blastophaga is supposed to produce on the fruit are of a botanical rather than a entomological nature, we may briefly say that two views have been held: (1) that, as in the fruit of the cultivated fig, only female flowers are produced, the Blastophaga is necessary for their fertilisation and the subsequent development of the fruit; (2) that the Insects stimulate the fig by biting parts thereof or by burrowing in it, and so give rise to the processes that have as their result the edible fruit. There seems to be little doubt that the Insect agency is necessary to the fertilisation of some species of figs. Cunningham, who has recently carried out an elaborate investigation as to the fertilisation of Ficus roxburghii,[[464]] concludes that in this fig, and probably also in other kinds, the perfect development is dependent on the access of the fig-Insects to the interior of the receptacular cavity. Should access fail to occur, both male and female flowers abort, without the formation of pollen grains by the former or seeds by the latter. The access of the Blastophaga is thus as necessary for the perfect evolution of the normal male and female flowers as it is for that of the modified ♀ or gall-flowers, with their contained ova and Insect-embryos. Whether the successful fertilisation of the flowers is really essential to the production of the edible fig is not a question for our discussion.
Fig-Insects are apparently more numerous in South America than they are in any other part of the world; and Fritz-Müller has discovered[[465]] a number of species there of a very extraordinary character, several of them possessing two forms of the male, one winged like the female, the other wingless and so different in character that they were considered to belong to a different genus. The wingless male of a species found in Madagascar, Kradibia cowani, has the peculiarity of possessing only four legs, the middle pair being represented merely by minute two-jointed rudiments. Some of these Insects live in galls on the figs. The fig-Insects were formerly considered to belong to the Proctotrypidae or to the Cynipidae (gall-makers), but there can be no doubt, notwithstanding they differ so much in their habits from the parasitic Chalcididae, that they probably belong to the same family. If treated as different from Chalcididae, they should be separated as a distinct family rather than united with the Cynipidae.[[466]]
It is impossible for us to do more than allude to the extraordinary shapes exhibited by some Chalcididae. The genus Thoracantha is specially remarkable in this respect. T. latreillei is said to resemble a beetle of the family Mordellidae, and has the wings concealed by false wing-cases—really projections from the thorax—so that from above the Insect bears no resemblance to the other Insects of the Order it really belongs to.
Fig. 359.—Thoracantha latreillei. Bahia. A, Upper, B, lateral aspect. (After Waterhouse.)