INSECTS. 207 



voracious. The mud-wasps, sand-wasps, and their allies, pro- 

 vision the nests for their young with other insects, destroying 

 in this way multitudes of injurious larvaj and winged insects 

 as well. One of our largest sand-wasps even captures the ci- 

 cada and stores it up as food for its- young ; another species 

 captures grasshoppers ; another seams to prefer the canker- 

 worm, and so on. The sting of all these wasps which store 

 up insects for their young, has the peculiar property of paral- 

 yzing the victims without destroying their life, so that they 

 live a long time, a supply of fresh living food for the young 

 wasps. All tlie dragon-flies and their allies feed exclusively 

 on other insects, devouring vast numbers of the smaller 

 forms. 



But the class of parasitic species is, of all, by far the most 

 beneficial. They attack every injurious species in some or all 

 its stages of growth. No species is free from their attacks. 

 The army-worm, for example, is known to be attacked by at 

 least nine different species of true insect parasites, two species 

 of two-winged flies, somewhat lilce house flies, and seven ich- 

 neumon flies, or their allies. Tliere are also large carnivorous 

 insects which prey upon them. The canker-worm is undoubt- 

 edly kept in check more by parasites and other insects which 

 prey upon it than by all its other enemies combined. The 

 most important of these parasites is a minute four-winged fly, 

 the little larva, or maggot, of which lives within the canker- 

 worm's egg. The female of this little fly, no bigger than a 

 pin's head, goes from egg to egg of the canker-worm, punc- 

 tures each with iier ovipositor, or sting, and deposits in it one 

 of her own eggs. From this little parasite's egg, too small to 

 be noticed by the naked eye, hatches a maggot which lives 

 within the canker-worm egg, where it finds food sufficient for 

 its sustenance till it is full grown, when it changes to a chrys- 

 alis, and finally into an adult which gnaws a hole in the can- 

 ker-worm egg-case and escapes a little fly like its parent, to 

 repeat like operations upon a fresh crop of canker-worm eggs. 

 These little parasites of course prevent the hatching of the 

 eggs which have served for their snug habitation, and, as 

 they are very abundant, they do a great service in destroying 



