5 I 6 HYMENOPTERA 



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 occur- 

 ring 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 



