UTILITY OF INSECTS. 269 



removing with great rapidity what might otherwise 

 prove nuisances of considerable magnitude, naturally 

 leads us to notice another sort of larva, no less useful 

 in diminishing the numbers of the plant-lice {Aphides) 

 which do so much damage to cltivated vegetables. 

 We do this also the more readily, that these very 

 insects, which are so beneficial to the husbandman 

 and the gardener, are often erroneously accused of 

 being themselves the cause of the mischief. A corre- 

 spondent of the Natural History Magazine, for exam- 

 ple, says, " the lady-bird is remarkably abundant this 

 season. The shrimp (larva) of this insect destroys 

 both turnips and peas in many parts of England."* 

 The truth is, however, that all the species of lady- 

 birds {Coccinellidce, Latr.), both in the larva and the 

 perfect state, feed exclusively on aphides, and never 

 touch vegetable substances. The eggs are usually 

 placed in a group of twenty or more upon a leaf, 

 where aphides abound ; and when the young are 

 hatched they find themselves in the midst of their 

 prey. There are a considerable number of species of 

 this family (Mr. Stephens enumerates fifty); but the 

 most common, perhaps, is the seven-spotted lady- 

 bird ( Coccinella scptempundata) , whose larva is of 

 considerable size, and, of course, when abundant, 

 must destroy a vast number of aphides. 



The maggots of many species of a beautifiil family 

 (SyrphidfS, Leach) of two-winged flies are also 

 voracious devourers of the aphides. These larvss are 

 of a tapering form, and they can contract or lengthen 

 their bodies to a considerable extent ; while they have 

 a retractile instrument, armed with three prongs like 

 a trident, with which they transfix their helpless and 

 hapless victims. " When disposed to feed," says 

 Kirby, " he fixes himself by his tail, and being blind, 



* Mag. of Nat. Hist., i. 191. 



