ARACHNIDA. 459 



The TapitelcB spin great webs of a close texture like hammocks, and 

 wait for the insects that may be entangled therein. The OrhitelcB 

 spread abroad webs of a regular and open texture, either circular or 

 spiral^ and remain in the middle, or on one side, in readiness to 

 spring upon an entangled insect. The Retitelm spin webs of an 

 open mesh-work, and of an irregular form, and remain in the middle 

 or on one side, to seize their prey. Lastly, the AquitelcB spread 

 their silken filaments under water to entrap aquatic insects. 



The silken secretion of spiders is not applied only to the formation 

 of a warm and comfortable dwelling for themselves, or of a trap for 

 their prey; it is often employed to master the struggles of a re- 

 sisting insect, which is bound round by an extemporary filament, 

 spun for the occasion, as by a strong cord. It forms the aeronautic 

 filament of the young migratory brood. It serves to attach the 

 moulting Hydrachna to an aquatic plant by the anterior part of the 

 body, when it struggles to withdraw itself from its exuvium. Lastly, 

 a softer and more silken kind of web is prepared for the purpose of 

 receiving the eggs, and to serve as a nest for the young. 



The lower forms of the Arachnids, e. g. Macrobiotus and AcaridcF, 

 are remarkable for their great longevity and their power of retaining 

 latent life. Some spiders live only twelve months ; others, e. g. 

 Segesti^ia senoculata, do not reach maturity until two years old. 



The Macrobiotus is androgynous, and the only known arachnid 

 that is so. The testes are two long fusiform sacs, situated one on 

 each side of the single ovarium and of the intestine ; they com- 

 municate with a median dorsal vesicula seminalis, in which Doyere 

 and Dujardin have detected actively moving spermatozoa. The 

 ovarium is a large sac, with loose and dilatable tunics, situated 

 dorsad of the intestine and advancing, when gravid with ova, 

 as far forwards as the first segment of the trunk. A short 

 oviduct opens at the fore part of the cloaca, which is on the ventral 

 aspect of the penultimate segment. The ovarian sac is sustained 

 by two suspensory ligaments or muscles, which diverge to be 

 attached, above the gastric division of the alimentary canal, to the 

 internal dorsal muscle of the second segment. The ova, which are 

 usually five or six, rarely more than ten in number, are simul- 

 taneously developed, and of large proportional size. The clear ger- 

 minal vesicle is imbedded in a coloured yolk, enclosed in a membrana 

 vitelli. When oviposition and moulting go on together, and the cast 

 skin receives the eggs, the chorion is smooth : at other times the 

 chorion is beset with points or tubercles. The germ-mass is trans- 

 formed at once into the Macrobiotus ; the young animal moves on 

 the twentieth day, and is excluded on the twenty-fourth, unless it 



