A FOREIGN INVASION OF ENGLAND 307 



fully expanded, their jointed legs weak and groggy. 

 But after a time, as they breathe or inflate them- 

 selves with air, all these parts grow fuller, lighter, 

 and harder. The Hessian fly in this predicament 

 waves her wings to and fro several times across 

 her back ; and in about a quarter of an hour 

 they have plimmed out fully, so that she can 

 soar away on her marriage-flight to meet her pro- 

 spective aerial husband. As for the tiny silvery 

 shroud or deserted pupa-case, it is left protruding 

 from the stem of the barley. 



This that I have given you is the history of a 

 successful and fortunate fly ; but not every indi- 

 vidual of the species is quite so lucky. As in the 

 case of the mosquito, nature at times makes not a 

 few failures. Sometimes the flies have insuperable 

 difficulty in freeing themselves from their articu- 

 lated coverings ; sometimes they break or spoil 

 their legs or wings, and become helpless cripples. 

 Yet so strong is the impulse of every species to 

 fill the world with its like that sometimes, says 

 Mr. Knock, even these poor maimed insects will 

 manage to crawl to a proper food-plant, and will 

 lay their eggs on it bravely like their more fortu- 

 nate sisters. He noted one crippled female which 

 in spite of its feebleness was eighty times over a 

 happy mother. This is usually the case with such 

 small insect pests ; their life consists, indeed, of 

 two things only, eating their way to the winged 

 stage, and then laying as many eggs as possible, to 

 do like damage in the next generation. 



Three or four hours after emerging, when they 



