358 ARACHNIDA—ARANEAE CHAP. 
Working upon a water plant some distance below the surface, it 
forms a silken dome of closely-woven threads, which it next 
proceeds to fill with air. To do this the spider rises in the 
water, raises its abdomen above the surface, and jerks it down 
again quickly, so as to carry with it a bubble of air which it 
helps to retain with its hind legs. With this it swims back 
to its tent, into which it allows the imprisoned air-globules to 
escape. By degrees the dome or bell is filled, and the creature has 
a dry and snug retreat beneath the water. In this it passes the 
winter in a torpid condition. The young of this species appear 
to be fond of utilising the empty shells of water-snails, which 
they float by filling them with air, and thus save themselves the 
trouble of nest-construction. 
Cocoon.—The last important spinning operation which remains 
to be described is the building of the so-called cocoon. This must 
be distinguished from the cocoon of insects, which is a protective 
covering of silk within 
which the larva assumes 
the pupa form. In the 
ease of the Spider, the 
term is applied to the 
structure which serves to 
protect and conceal the eges. 
It is often of considerable. 
complexity, and is highly 
characteristic of the parti- 
cular species which con- 
structs it. 
All egg-bags are com- 
menced in very much the 
ie same way. A small sheet 
Fo 196. Bag cocoons A cia diadenele:_of silk is woven, and against 
to a leaf. ©, Agroeca brunnea, nat. size, this, sometimes upon the 
attached to a weed, and not yet coated with : 
mud. D, Hro furcata x 4, attached to a log. upper and sometimes on the 
under surface, the eggs are 
deposited, and then covered in with a second silken layer. The 
compact silk-covered ball of eggs is then, in many cases, enclosed 
in a small compartment which the spider builds with infinite care 
and unfailing uniformity, after the pattern peculiar to its kind. 
A considerable number of the Orb-weavers are content with a 
