TETRAPNEUMONES. 647 



body, thereby tightening the web in every direction so that the 

 vibration is prevented. 



" The construction of nets for catching food is not the only 

 use of the tliread made by these spiders. They seldom move 

 from place to place without spinning a line after them as they 

 go. They are able b}^ its use to drop safely from any height, 

 and when suspended by it are carried by the wind across wide 

 spaces without any exertion on their part, except to let out the 

 thread. The crevices in which they pass the winter and the 

 leisure hours of summer, are partly lined and enclosed by a 

 coating of silk resembling that used for confining captured in- 

 sects. The eggs are enclosed in a cocoon of the same mate- 

 rial, and there the young remain until they are strong enough 

 to shift for themselves, growftig to nearly double their size 

 without apparent nourishment. 



"Several hundred j^oung are produced by a single female, 

 but probably it is seldom that one-tenth of this number ever 

 reach adult size. Nearly all the spiders which we see in webs 

 are females or young. They spend most of their time in the 

 vicinity of their webs, and many doubtless pass their lives 

 within a few yards of the place of their birtli. The adult males 

 are seldom seen building or occupying webs : they remain con- 

 cealed during the day, and at night wander about from web to 

 web. When young there is no obvious difierence between the 

 sexes, but as the time for the last moult approaches, the ends 

 of the palpi of the male swell to several times their former 

 size. When the time for the final moult arrives, both sexes 

 retire to their holes and cast off the skins of their entire bodies, 

 even to the claws. This process obliges them to remain con- 

 cealed until the new skin has acquired suflflcient strength and 

 firmness, when they again return to their webs. The females 

 still resemble the young, except in size, but the males are 

 distinguished from them by the greater length of their limbs, 

 the diminished size of the posterior half of the body, and the 

 large and complicated joints of the palpi (Plate 11, fig. 8)." 



Tetrapneumoxes Latreille. The large hairy species of 

 Mygale differ from other spiders in having four lung-sacs and 

 as many stigmata, and only two pairs of spinnerets, of which 



