The great resemblance of the inquiline gall-fly to the fly that makes the gall both dwell in, has been several times noticed by Osten Sacken, who says "one of the most curious circumstances connected with the history of two North American blackberry galls is, that besides the Diastrophus, which apparently is the genuine originator of the gall, they produce another gall-fly, no doubt an inquiline, belonging to the genus Aulax, and showing the most striking resemblance in size, colouring, and sculpture to the Diastrophus, their companion. The one is the very counterpart of the other, hardly showing any differences, except the strictly generic characters! This seems to be one of those curious instances, so frequent in entomology, of the resemblance between parasites and their hosts! By rearing a considerable number of galls of D. nebulosus I obtained this species as well as its parasite almost in equal numbers. By cutting some of the galls open I ascertained that a single specimen of the gall frequently contained both species, thus setting aside a possible doubt whether these Insects are not produced by two different, although closely similar galls."[[438]]
The substance of which galls are composed, or rather, perhaps, a juice they afford, is apparently a most suitable pabulum for the support of Insect life, and is eagerly sought after by a variety of Insects; hence by collecting galls in large quantities many species of Insects may be reared from them; indeed by this means as many as thirty different kinds of Insects, and belonging to all, or nearly all, the Orders, have been obtained from a single species of gall. Some galls are sought by birds, which open them and extract their tenants, even in cases where it might be supposed that the nauseous flavour of the galls would forbid such proceedings.
Not more than 500 species of Psenides and Inquiline Cynipidae are known from all parts of the world; and of described Parasitic Cynipidae there are only about 150 species. The British forms have recently been treated by Cameron in the work we have already several times referred to.[[439]]
A few Cynipidae have been found in amber; and remains of members of the family, as well as some galls, are said by Scudder to have been found in the Tertiary strata at Florissant.
Fam. II. Proctotrypidae, or Oxyura.
Small Hymenoptera, with few, or even no, nervures in the wings: the pronotum closely adherent to the mesothorax, and at the sides reaching backwards to the points of insertion of the wings. The abdomen is pointed, and the pointed apex is frequently deflexed; the ovipositor is not coiled, but is retractile, and when extruded is of tubular form, and apparently a continuation of the tip of the body. The earlier stages are passed in the bodies, or in the eggs, of other Arthropods.
Fig. 351.—Helorus anomalipes. Britain.
The Proctotrypidae is one of the most difficult groups of Hymenoptera to define; some of its members exhibit a great resemblance to Aculeate Hymenoptera. This is the case with the Insect we figure (Fig. 351). It, however, is an undoubted Proctotrypid, but there are other forms that approach very closely in appearance to the Aculeata, or stinging Hymenoptera; so that until a better comprehension is reached as to the distinction between a sting and an ovipositor the separation between Proctotrypidae and Aculeata must be considered somewhat arbitrary.
There is extreme variety in the family; the wings differ considerably in shape and neuration; they are not infrequently altogether absent in one or both sexes. The chief distinction of the family from other parasitic Hymenoptera is the tubular form of the ovipositor; which part appears to be a continuation of the tip of the body. This latter is more definitely acuminate than usual, and has given rise to the term Oxyura, by which name the Proctotrypidae are distinguished in many books. From the Chalcididae they are distinguished also by the angles of the pronotum attaining the tegulae. In this character they agree with the Cynipidae, but the ovipositor and abdomen are very different in form in these two groups, and the Proctotrypidae very frequently have a pigmented spot or stigma on the front wings which is absent in Cynipidae. As if to add to the difficulties the systematist meets with in dealing with this family, some of its members have the trochanters undivided, as in the case of the stinging Hymenoptera. The larvae of all that are known lead a completely parasitic life in the bodies or eggs of other Insects or of Spiders. Sometimes half a dozen specimens may find the means of subsistence during the whole of their development in a single Insect's egg. Usually Proctotrypids pupate in the position in which they have fed up, enclosed each one in a more or less distinct cocoon. In Fig. 352 we represent a very remarkable case of Proctotrypid pupation; a larva of some beetle has nourished many specimens of a species of the genus Proctotrypes, and the pupae thereof project from the body of the host, a pair of the parasites issuing from each segmental division in a remarkably symmetrical manner.