xiv ENEMIES OF SPIDERS 367 



their silken envelope, are well protected against the severities of 

 the weather. 



A more universal enemy to the egg is found in Ichneumon 

 flies. On examining the cocoons of almost any species of 

 spider, a large proportion are almost certain to be found to 

 contain Ichneumon larvae. Mr. F. Smith, in the Transactions 

 of the Entomological Society for I860,, describes two species, 

 Hemeteles fasciatus and H.formosus, which are parasitic on the 

 eggs of Agelena brunnea They are figured in Mr. Black wall's 

 book on British Spiders. Pezomachus gracilis attacks the cocoons 

 of many kinds of American spiders, appearing to have no special 

 preference for any particular species, while Acoloides saitidis 

 seems to pay special attention to the eggs of certain of the 

 Jumping-spiders, and particularly of Saitis pulex. 



The Ichneumons which thus regard the Spider's eggs as con- 

 venient food for their own larvae are probably very numerous, 

 Nor are they themselves always free from parasites. Occasionally 

 the larvae of minute Hymenopterous insects are found to be 

 parasitic upon the eggs of an Ichneumon which have been laid in 

 a Spider's cocoon. 



It sometimes happens that the development of the young 

 spiders has so far advanced at the time of the Ichneumon's 

 intrusion that the latter's intention is frustrated, and its offspring, 

 instead of devouring, are themselves devoured. Again, some few 

 of the eggs in an infested cocoon occasionally escape the general 

 destruction and reach the adult condition, but there can be no 

 doubt that Ichneumons are largely instrumental in keeping down 

 the numbers of most species of spiders. The perils which attend 

 the Spider after leaving the cocoon are no less formidable, and 

 much more numerous. The whole newly -hatched brood may be 

 destroyed by a heavy rain -storm. If there is not a sufficient 

 supply of food suitable to their feeble digestive powers they 

 perish of inanition, or eat one another. This cannibalistic pro- 

 pensity is a considerable factor in the mortality among young 

 spiders, and the adult animals frequently prey upon one another. 



Argyrodes piraticum, in California, invades the webs of larger 



spiders of the family Epeiridae, which it seizes and devours. A. 



trigonum, common in the eastern states of North America, has the 



same habit. 1 Hentz found in Alabama a spider, which he named 



1 M'Cook, t.c. p. 389. 



