DIPTERA. 



49 



much more pronounced, as for instance in the stag-horned flies, in which the head 

 of the male is furnished with large branching processes, and the stalk-eyed flies, in 

 which the eyes in this sex are supported upon long, horizontal, immovable stalks. 



Like the other higher orders of insects, flies, in the course of their development, 

 go through a complete metamorphosis ; the larvae — of which perhaps the commonest 

 are maggots and cheese-hoppers — being worm-like, and passing into a partially or 

 wholly quiescent pupal stage before attaining maturity. These larvae differ much 

 in structure in some of the families ; those of the gnats having a well-developed 

 head, with the antennae, mandibles, maxillae, and labium always recognisable ; 

 whereas in the maggots of the blow-fly the head is narrow and pointed, without 

 antennae, and with the mouth -parts reduced to a pair of retractile hooks, the 

 opposite extremity of the body being broad and square-cut. It must not be supposed, 

 however, that the larvae of all the members of this order are of one or other of 

 these two types. On the contrary, the structure varies according to habitat, and 

 almost every gradation is found linking the two together. Some species live in 

 fresh- water ponds and streams, others in the earth amongst roots of grass, others 

 again in rotting animal or vegetable matter, and others, like the maggots of the 

 warble-fly, in the stomachs of the hosts they infest. Thus the nature of their food 

 and surroundings is extremely varied, and that the larvae are likewise so, may be 

 seen by a glance at the figures in the following pages. 



Upon reaching its full size the larva passes into the pupal stage. The pupa, 

 however, exists under two conditions. In one case, as in the gnats, it emerges from 

 the skin of the larva and leads an independent life of longer or shorter duration, 

 until the attainment of maturity ; in the others, as in the fly called Stratiomys, it 

 remains within the larval skin, which becomes thickened and constitutes a 

 protective covering for it. Again, the rupture of the larval skin for setting free 

 the pupa is effected in one of two ways. In the first case the opening is T-shaped, 

 consisting of a longitudinal split on the back behind the head, or rarely of a transverse 

 split between the seventh and eighth segments of the body ; in the second case a 

 circular split occurs behind the head, which is pushed off like a kind of cap. These 

 two methods of splitting of the larval skin have been used as characters for 

 dividing the Diptera into two suborders, those in which the pupa escapes in the 

 former way being termed straight-seamed flies, or Orthorrhapha, and those in which 

 the pupa escapes in the latter way circular-seamed flies, or Cyclorrhapha. For the 

 rupture of the larval skin, the pupae of the Cyclorrhapha are furnished with a bladder- 

 shaped excrescence on the front of the head. In the vast majority of flies the 

 young make their first appearance in the form of eggs. In some few cases, 

 however, as in the genera Sarcophaga and Mesembrina, belonging to the family 

 Muscidce, the young are born as active maggots ; while in the forest-flies and their 

 allies only one young one matures at a time, and this is retained by the mother and 

 nourished at her expense until it has passed into the pupal stage. The most 

 anomalous method of reproduction occurs in one of the gall-midges, where the 

 larvae themselves produce other grubs by a process of internal budding. 



That flies were abundant in early Tertiary times, when they were not very 

 different from those that now exist, is shown by the abundance of their remains 

 preserved in the amber beds of the Baltic. Strata of the same age at Florissant, 



VOL. VI. — 4 



