70 THE LIFE-STORY OF INSECTS [CH. 



the cuticle ; the hindmost spiracles are often larger 

 than the others. These little grubs live in family com- 

 munities, their presence leading to some deformation 

 of the plant that serves to shelter them. A shrivelled 

 fruit or an arrested and swollen shoot, such as may 

 be due respectively to the Pear-midge (Diplosis pyri- 

 vora) or the Osier-midge (Rhabdopliaga heterobia), 

 is a frequent result of the irritation set up by these 

 little grubs. In a larva of the crane-fly family (Tipu- 

 lidae, fig. 20) living underground and eating plant- 

 roots, like the well-known 'leather-jacket' grubs of the 

 large 'Daddy-long-legs' (Tipula) or burrowing into a 

 rotting turnip or swollen fungus, like the more slender 

 grub of a 'Winter Gnat' (Trichocera), the student 

 notices a somewhat tough cuticle, a relatively small 

 but distinct head, and frequently prominent finger-like 

 processes on the tail- segment. Further examination 

 shows a striking modification in the arrangement of 

 the spiracles. Instead of a paired series on most of 

 the body-segments, as in caterpillars and the vast 

 majority of insects whether larval or adult, there are 

 two large spiracles surrounded by the prominent 

 tail-processes, and a pair of very small ones on the 

 prothorax, the latter possibly closed up and useless. 

 This restriction of the breathing-holes to a front and 

 hind pair (amphipneustic condition) or to a hind pair 

 only (metapneustic type) is highly characteristic of 

 the larvae of Two- winged flies. 



