92 
Guide to Arachnida. 
Table- Mention must also be made here of the curious sound-producing 
^ se | g Nos ' organs which are found in many of the Theridiidae. These spiders 
have the anterior part of the abdomen especially hollowed out and 
hardened, the surface of this concavity being armed with teeth or 
ridges which can be moved against the granular or striated surface 
of the posterior end of the cephalothorax. In several of the 
Agelenidae also an analogous structure occurs, but the structures 
on the abdomen are rubbed against an enlarged tooth-like projec- 
tion, which is present on the pedicel separating the cephalothorax 
from the abdomen. 
Spiders are oviparous. They construct a “ cocoon ” (or several 
cocoons) for the protection of the eggs, and this usually consists of 
several layers of silk, the outermost coat in many cases being of 
especial strength or thickness. Many species seem to give but little 
or no attention to their cocoon when once it has been completed. 
Very often, however, the mother watches over it with extreme 
solicitude until the young spiders emerge, and displays great 
courage in its defence in times of danger. Special tents or cells 
of silk for the reception of the cocoon are constructed by many of 
the spiders which lead a wandering life, and by the tube-spinning 
spiders (see Wall-case 7). In these cases the mother shuts herself 
up with the cocoon, remaining within on guard until the eggs 
hatch. A large number of spiders which lead a predatory life 
(Lycosidae, etc.) carry the cocoon about with them, either in their 
chelicerae or attached to their spinnerets. 
The dispersal of the young of Araneids, which usually takes place 
during the early part of the summer or in the autumn in this 
country, is greatly helped by their aeronautic habits. The young 
spider climbs to the top of a shrub or other point of vantage and 
turns its face in the direction from which the wind is blowing. 
It then proceeds to straighten its legs, standing on the tips of them 
and elevating its abdomen in the air. One or more threads of silk 
now make their appearance, issuing from the spinnerets, and are 
drawn out by the wind into long floating lines. At length the 
spider lets go and is wafted away through the air, supported by its 
air-ship of threads. In his “ Naturalist’s Voyage ” Darwin makes 
the following interesting observation on the ballooning habit of 
spiders : “ On several occasions, when the Beagle has been within 
the mouth of the Plata, the rigging has been coated with the web 
of the Gossamer Spider. One day (November 1st, 1832) I paid 
particular attention to this subject. The weather had been fine 
and clear, and in the morning the air was full of patches of 
