ENEMIES AND THEIR INFLUENCE. 391 



Agalena and the mazy annex of Labyrinth ea, or the foundation lines of 

 Orbweavers, wliich somewhat resemble her own retitelarian forms. The 

 same remark applies to Argyrodes piraticum. It is found upon the com- 

 pound snare of Zilla, which combines the orbweb with the retitelarian, 

 and stations itself for its piratical raids upon the netted outlying and 

 foundation lines of other Orbweavers. The same observation holds good 

 as to Argyrodes trigonum, as far as its nesting parasitism is concerned, 

 for she finds congenial raiding grounds within the netted lines that over- 

 hang the funneled sheet of Agalena and the orb of Labyrinthea. Un- 

 doubtedly she feels more courage, confidence, and vigor within an environ- 

 ment which gives her the sense of being upon her native snare. 



It is possible for us to conceive that the habit may have originated 

 from the facility thus presented for obtaining a foothold and home upon 

 the webs of its neighbors. Having done this, it only needed a favoring 

 opportunity, many of which must have presented, to throw the host of 

 the web into the power of its guest. An act so advantageous would be 

 likely to be repeated and persisted in ; we have thus an easy way to the 

 development of a fixed habit from what at first may have been an acci- 

 dental feature in the life of ancestors. Even if this conjecture should be 

 adjudged ]ilausible, I cannot free myself from the wonder that so mani- 

 festly convenient a mode of securing food should have been fixed upon by 

 so few of all the numerous species and innumerable individuals of the 

 great Retitelarian tribe. 



VI. 



We pass now to note the parasitic enemies of Orbweavers and others 

 of their order. Mr. Blackwall has shown that immature spiders, Epei- 

 roids and others, are infested by the larva? of Polysphincta car- 

 Parasites ^^*^"'^''i'^' '^ hymenopterous insect belonging to the Ichneumonidae. 

 This parasite is always attached to the upper part of the abdo- 

 men, near its union with the cephalothorax, and, although it proves a 

 source of constant irritation, is secured by its position from every attempt 

 of the spider to displace it. Being witliout feet, it appears to retain its 

 hold upon its victim solely by the instrumentality of the mouth and a 

 viscid secretion emitted from its caudal extremity. But one larva has ever 

 been seen upon a single spider. 



The ichneumon probably deposits its eggs on the spiders in the au- 

 tumn, attaching one egg to each individual. In the spring, towards the 

 end of May, having gone through its final moult and increased consider- 

 ably in size, the larva becomes restless and rapidly destroys the spider, 

 which it abandons after having reduced it to a mere corrugated skin. It 

 then attaches itself to some convenient point, the cork of a bottle if it' be 

 in confinement, and begins to spin its cocoon, which it completes in a day 

 or two. This cocoon is of yellowish white silk of compact texture, and 



