i82 THE BIOLOGY OF INSECTS 



skeleton ; c, ridge on plate C of pharyngeal skeleton ; D, bridge plate of 

 pharyngeal skeleton in roof of atrium ; DiMcl, dilator muscles of 

 pharynx; DP, dorsal pouch of atrium, divided beyond base into two 

 wings containing plates C of pharyngeal skeleton and leading to roots 

 of frontal sacs ; DPMcl, dorsal protractor muscles of pharynx ; EMcl, 

 extensor muscles of oral hooks ; FMcl, flexor muscle of oral hook ; FS, 

 frontal sacs, containing imaginal buds of antennae and compound eyes ; 

 GC, gastric caecum ; Gng, ventral ganglionic nerve mass ; g, h, sensory 

 papillae of snout of larval head; Hk, mouth-hooks; Lj, Lg, L3, leg 

 buds ; LbB, imaginal buds of labium ; LH, larval head ; LMy labrum ; 

 LPMcl, lateral protractor muscles of pharynx; Mth, larval mouth; ofe, 

 imaginal bud of compound eye ; (E, gullet ; Phy, lumen of larval pharynx ; 

 PSp, posterior spiracle; Pvent, proventriculus ; SalD, salivary duct; 

 »Sa/G/, salivary gland ; F^f, stomach. From R. E. Snodgrass (Jowrw. 

 Agr. Res. U.S.D.A. xxviii, 1924). 



front end, where there are paired sensory tubercles, and 

 strong mouth-hooks used for tearing, which can only with 

 much doubt be compared with typical insect mandibles. 

 An interesting peculiarity of the muscoid maggots is the 

 restriction of spiracles to a large pair at the tail-end of 

 the body and a small pair on the prothorax which can have 

 but a very restricted function (Fig. 51, PSp, ASp). In 

 some of these maggots several pairs of the lateral spiracles 

 have been detected in a vestigal condition, their connect- 

 ing air-tubes excessively slender and solidified by internal 

 deposition of cuticle. 



These larvae of wasps, bees, and muscoid flies, which 

 differ — especially the last — so profoundly from their parents, 

 are adapted each to its characteristic mode of life. The 

 wasp-grub rests in its paper chamber (Plate VII) in the nest 

 where it is fed on insect-fragments by its sisters the worker- 

 wasps, and the bee-grubs in the waxen chambers of the comb 

 are provided by the worker-bees with floral food materials 

 such as honey and pollen. The soft defenceless cuticle, 

 the small head and relatively weak jaws are enough for 

 creatures that are protected and provided for, have no need 

 to flee from enemies nor to wander in search of food. A 

 remarkable feature of these hymenopterous grubs is that 

 throughout the larval stages the hind intestine is closed 

 and no waste matter passes from the food-canal until just 

 before pupation ; this seems a suitable adaptation in view 



