ARACHNIDA. 631 



In the genus Hersilla we see clearly that the three pairs of 

 spinnerets are but modified legs. The second and inner pair 

 are generally the smallest, while the third and largest pair are 

 the most posterior. Their office is to reel out the silk from the 

 silk-glands. The tip of the articulated spinnerets ends in a 

 cone, perforated by myriads of little tubes (over 1,000 in 

 Epeiira, about 300 in Lycosa, and a less number in the smaller 

 species) through which the silk escapes in excessively delicate 

 threads, which unite to form the common thread visible to the 

 naked eye. (Plate 10, fig. 4, spinnerets of Epe'ira vulgaris en- 

 larged twenty-five diameters ; fig. 5, a spinning tube.) 



The Acarina are supposed to have glands analogous to the 

 silk glands, whose product, like silk, hardens on exposure to 

 the air, and by which certain parasitic genera, such as Uro- 

 poda, fix themselves solidly to their host. Siebold states also, 

 that "many species of Hydrachna fix, by a kind of glue, the 

 anterior portion of their body on aquatic plants, and in this 

 position await the completion of their moulting. The organs 

 secreting this substance have not yet been discovered. It is 

 well known that the European Tetraiiychus telarius spins 

 large webs on the leaves of trees and on house-plants. 



The reproductive system is much as described in insects, ex- 

 cept that the external appendages are rarely developed in 

 either sex. The genital armor is situated at the base of the 

 abdomen ; it is concealed when present under the skin. 



In the Acarina the two ovaries open on the middle of the 

 abdomen, or on the under side of the thorax, either between 

 or behind the last pair of legs. In Hydrachna the oviduct 

 opens into an ovipositor by which the insect is enabled to lay 

 its eggs under the skin of the fresh-water mussel on which it is 

 parasitic, and other mites oviposit in a similar way under the 

 epidermis of plants. 



In most spiders the two ovaries have their outlet in an ori- 

 fice situated between the two lung-sacs. They have a distinct 

 receptaculum seminis, especially marked in Epeira. "The 

 Scorpionidce have three ovaries, consisting of as many lon- 

 gitudinal ones, united by four pairs of transverse one's." The 

 outer two of the former are oviducts, leading out at the base 

 of the abdomen. 



