THE HIDDEN TYPE OF WING-GROWTH 133 



the ventral aspect, beset with numerous fine bristles which 

 assist the maggot's crawling action. 



A flattened dorsal area (Fig. 75, g) of the hindmost segment 

 slopes steeply backwards and, like the corresponding region 

 in the crane-fly grub, carries the pair of large tail-spiracles. 

 These are circular in outline, each with three narrow slits 

 through which air passes in and out of the breathing tubes. 

 From these tail-spiracles the main air-trunks run forward 

 along the sides of the body, becoming narrower towards the 

 head-region and terminating in a pair of small anterior spiracles 

 (Figs, 75 c, 76 6&) situated at the sides of the first body- 

 segment behind the anterior mouth-bearing region. These 

 spiracles, minute and apparently of little functional importance, 

 are remarkable in their form ; the front end of the air trunk 

 divides into eight or ten little tubes divergent but remaining 

 in contact with each other, the whole structure showing as a 

 beautiful little fan-like object against the white surface of the 

 larva. In connexion with the extreme reduction in the number 

 of spiracles among these fly larvae it is of great interest to notice 

 that in a few muscoid maggots 1 a series of five or six pairs of 

 vestigial abdominal spiracles has been detected, these minute 

 openings in the cuticle being connected with the tracheal system 

 by air-tubes which become solidified by the filling up of their 

 cavities (Fig. 77) . As in the crane-fly's grub, the spiracular area 

 on the blow-fly maggot's tail-segment is surrounded by regularly 

 arranged, conical, finger-like processes, and the somewhat 

 ventrally situated anus has a blunt " proleg " on either side. 

 In the successive stages of such a maggot the mouth-hooks 

 and the tail-spiracles show an advance in the complexity of 

 their structure (Fig. 76). 



This fly-maggot is so important a type of insect larva that 

 a brief account of its internal structure, such as has been given 

 in some previous cases, seems desirable. In the digestive 

 system, the pharynx with its chitinized lateral walls is 

 succeeded by a tubular gullet (Fig. 78 oe) from the forward 

 region of which is given off a bladder-like sucking sac. The 

 gullet expands into a small crop (c), the hindmost portion of 



1 L. C. Miall and T. H. Taylor : " The Structure and Life History of 

 the Holly-fly ". Trans. Entom. Soc., Lond. 1907. G. H. Carpenter and 

 F. J. Pollard : ' The Presence of Lateral Spiracles in the Larva of 

 Hypoderma ". Proc. R. Irish Acad., XXXIV. 1917. 



