3 oi2 . THE JOINTED ANIMALS 



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 develop- 

 ment, 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 recog- 

 nizable; whereas in the maggots of the blowfly the head is narrow and pointed, 

 without antennas, 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 habi- 

 tat, and almost every gradation is found linking the two together. Some species 

 live in fresh-water ponds and streams, others in the earth among 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 protect- 

 ive 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, consist- 

 ing 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 characteristics 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 flys or Cyclorrhapha. For the rup- 

 ture 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, how- 

 ever, 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 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 them- 

 selves 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 



