In Brazil Dielocerus ellisii, a sawfly allied to Hylotoma, constructs a nest in which the cocoons of many specimens are crowded together, being packed side by side like the cells in the comb of the bee, while the whole mass is protected by a thick outer wall. It is not known in what manner this communal work is carried out, but it is interesting to note that the cocoons assume to a considerable extent the hexagonal form of the cells in the comb of the bee. Some doubt was expressed as to the interpretation put on this structure by Curtis, but his observations have been confirmed by Smith and Peckholt.
Several species of sawflies are known to be very injurious to crops. One of these—the sawfly of the turnip, Athalia spinarum (centifoliae Panz.)—sometimes commits excessive depredations on the turnip crops in this country as well as on the continent of Europe; its life-history and anatomy were described by Newport in an essay published by the Entomological Society in 1838. The eggs, it appears, are laid singly at the edges of the leaves in the month of May, as many as 200 or 300 being deposited by one female; as the parent flies are usually gregarious, appearing in large numbers in fields of turnips, it is not difficult to form an idea of the serious nature of their depredations. The egg grows very considerably; the development of the embryo is rapid, occupying, even in unfavourable weather, only seven or eight days, while in quite congenial circumstances it is probable that the eggs may hatch about the fourth day after their deposition. The young grub immediately begins to feed, and in about five days changes its skin for the first time; it repeats this operation twice at similar or slightly longer intervals, the third moult thus occurring when the larva is three or four weeks old; it is then that the larva begins to be most destructive. Sunshine and warm weather are very favourable to it, and under their influence it grows so rapidly that in a few days a field may be almost completely stripped of its foliage. This larva is of a sooty black colour, and will live on other Cruciferous plants quite as well as on the turnip. When full grown it buries itself to a slight depth under the surface of the earth, and forms an oval cocoon of a firm texture, and with many particles of earth closely adherent to it. The perfect fly emerges towards the end of July, and a second brood will be produced in the same season if circumstances are favourable; in that case the resulting larvae enter the ground for the formation of their cocoons in September or October, and pass the winter in their cocoons, but still in the larval state; changing to pupae in the following spring, and appearing as perfect Insects in May. From this account it appears not improbable that the offspring of a single female existing in the April of one year may amount by the following May—three generations having been passed through in the interval—to as many as 27,000,000 larvae. Fortunately the creatures are, as Frauenfeld observed, destroyed in very large numbers by a parasitic fungus and by a Nematode (Filaria).
We have, earlier in the chapter, alluded to the fact that the phenomena of parthenogenesis prevail somewhat extensively among sawflies. It is the rule in the family that males are very much less numerous than females, and there are some species of which no males have been discovered. This would not be of itself certain evidence of the occurrence of parthenogenesis, but this has been placed beyond doubt by taking females bred in confinement, obtaining unfertilised eggs from them, and rearing the larvae produced from the eggs. This has been done by numerous observers with curious results. In many cases the parthenogenetic progeny, or a portion of it, dies without attaining full maturity. This may or may not be due to constitutional weakness arising from the parthenogenetic state. Cameron, who has made extensive observations on this subject, thinks that the parthenogenesis does involve constitutional weakness, fewer of the parthenogenetic young reaching maturity. This he suggests may be compensated for—when the parthenogenetic progeny is all of the female sex—by the fact that all those that grow up are producers of eggs. In many cases the parthenogenetic young of Tenthredinidae are of the male sex, and sometimes the abnormal progeny is of both sexes. In the case of one species—the common currant sawfly, Nematus ribesii—the parthenogenetic progeny is nearly, but not quite, always, entirely of the male sex; this has been ascertained again and again, and it is impossible in these cases to suggest any advantage to the species to compensate for constitutional parthenogenetic weakness. On the whole, it appears most probable that the parthenogenesis, and the special sex produced by it, whether male or female, are due to physiological conditions of which we know little, and that the species continue in spite of the parthenogenesis, rather than profit by it. It is worthy of remark that one of the species in which parthenogenesis with production of males occurs—Nematus ribesii—is perhaps the most abundant of sawflies.
Although many kinds of Insects display the greatest solicitude and ingenuity in providing proper receptacles for their eggs, and in storing food for the young that will be produced, there are extremely few that display any further interest in their descendants; probably, indeed, the majority of Insects die before the eggs are hatched, one generation never seeing the individuals of another. It is therefore interesting to find that a fairly well authenticated case of maternal attachment, such as we have previously alluded to as occurring in earwigs, has been recorded in Perga lewisii, an Australian sawfly of the sub-family Cimbicides. The mother, having deposited about eighty eggs on the leaf of a Eucalyptus, remains with them until they hatch, after which she sits over her brood with outstretched legs, and with admirable perseverance protects them, so far as she is able, from the attacks of parasites and other enemies; she quite refuses to be driven away from her charges. Mr. Lewis, to whom we are indebted for this account,[[427]] states that the sawfly does not recognise her own special brood, but will give equal attention to another brood if she be transferred thereto; and he adds that many of the batches of larvae were destitute of any maternal guardian.
There are about 2000 species of sawflies known. A large majority of them are found in the European and North American regions; still, a good many are known to live in South America, and Perga—one of the genera of the family containing many species of large size—is peculiar to the Australian region. Although the family includes so many species, very few anomalies of structure have been detected in it; one species, Pompholyx dimorpha Freymuth, is described as being apterous in the female, and as having the thorax curiously modified in its form. There are no very small Insects in the family, and none over the middle size. Nearly 400 species have been detected in Britain; this number could certainly be increased by persevering researches. The palaeontological record has hitherto given only a very meagre evidence about sawflies. Several species have been preserved in amber, and three or four are known from Tertiary strata in Europe and North America.
CHAPTER XXIII
HYMENOPTERA PETIOLATA–PARASITIC HYMENOPTERA–CYNIPIDAE OR GALL-FLIES–PROCTOTRYPIDAE–CHALCIDIDAE–ICHNEUMONIDAE–BRACONIDAE–STEPHANIDAE–MEGALYRIDAE–EVANIIDAE–PELECINIDAE–TRIGONALIDAE.
We now pass to the consideration of the Hymenoptera of the sub-Order Petiolata, or Apocrita, as they are styled by Brauer. We should make use of the term Petioliventres, for it contrasts naturally by its termination with Sessiliventres, were it not that the word is so uncouth that we think it better to adopt the shorter and more euphonious expression, Petiolata.
The members of this sub-Order, without exception, have the hind body connected with the thorax by means of a deep constriction, so that the base of the abdomen (Fig. 336, B, b) is very narrow; the articulation between the two parts is effected by means of a complex joint allowing great play, and facilitating the operations of boring and stinging, processes that are of extreme importance in the economy of the great majority of the species. The petiole is sometimes extremely short, but it may be so long that it appears like a stalk, at whose extremity is borne the remaining part of the abdomen (Fig. 369). When the petiole is very short the abdomen reposes close to the back of the thorax (Fig. 331, C), and in this case the abdomen is usually described as sessile; while, when it is evidently stalked, it is said to be petiolate. These terms are, however, unsuitable, as the words sessile and petiolate should be reserved for the conditions characteristic of the two sub-Orders. We shall therefore use the terms pseudo-sessile and pedicellate for the two conditions of the Petiolata.
The Hymenoptera Petiolata comprises an enormous majority of the Order. Although it includes many of the most interesting and important of Insects, its classification is but little advanced, for a great many of the forms are still rare or unknown. Three series may be adopted for the purposes of nomenclature.