CHAP. XXI. BENEFICIAL INSECTS. 141 



The larvae of the Lace-winged Flies (Fig. 47) feed upon plant- 

 lice and similar insects. After reaching their full growth each 

 one spins, in some sheltered place, a globular cocoon, and 

 is soon after changing to a pupa; in due time the pupa comes 

 out of the cocoon and fastens itself to some neighboring 

 object, when the skin on its back next splits open and the per- 

 fect fly (Fig. 21) makes its escape. The female fly deposits 

 her eggs upon the tips of slender, thread-like stems (Fig. 21).. 



The larva of the Syrphus-fly (Fig. 231, a) also feeds upon 

 Plant-lice and similar insects. It is entirely destitute of legs, 

 and after reaching, its full growth attaches itself to the stem of 

 a plant, or some other object, and soon contracts to a pupa 

 (Fig. 231, 6), from which the perfect fly (Fig. 231, c), in due 

 time, makes its escape. When changing to a pupa the narrow 

 tapering end of the larva becomes the thickened end of the 

 pupa. 



Many kinds of Wasps, such as the Russet-red Wasp (Fig. 183, 

 a), provision their nests with caterpillars, flies, or other insects 5 

 to serve as food for their young. Some kinds build nests of 

 mud (Fig. 181); others of a papery substance (Fig. 183, 6); 

 still others build their nests in holes in the ground, in decayed 

 wood, or in the stems of plants. The larvae or young of these 

 Wasps are entirely destitute of legs. 



There is a group of insects which, in the larva state, live 

 within the eggs, or within the larvae or pupae of other insects? 

 ultimately destroying the latter; such are commonly called 

 "parasitic" insects. The Ichneumon-flies (Figs. 39, 40, 131, 

 and 134) belong to this class. The female deposits her eggs 

 in the larvae or pupae of other insects in which her progeny 

 are to live; from these eggs are hatched small footless grubs, 

 which feed upon the internal parts of the larva or pupa in 

 which the eggs, from which they were produced, had been 

 deposited by the provident mother. After reaching their full 

 growth some kinds assume the pupa form within the larva or 

 pupa in which they live, and the flies, when issuing from the 

 pupa, gnaw holes through the skin of the larva or pupa, and 

 thus make their escape. The larvae of several kinds, however, 



