iS 



ARACHNIDA ARANEAE 



CHAP. 



That these scuta are sometimes indicative of an obsolete 

 segmentation would seem likely from the study of the remarkable 



species, Tetrahlemma mediocu- 

 latum (Fig. 176), described by 

 Pickard Cambridge, from 

 Ceylon. In addition to large 

 dorsal and ventral scuta, tlie 

 sides and posterior extremity 

 are guarded by smaller scuta, 

 the disposition of vs^hich is 

 well seen in the figure. 



The normal smooth ab- 

 domen presents dorsally no 

 very striking features. In 

 species of variegated colora- 

 tion there is very generally 

 noticeable a median dentated 

 band (Fig. 173), the "normal 

 marking " of some writers, which would appear to have some 

 correlation with the underlying dorsal vessel. Beneath the 

 abdomen are to be seen the orifices of the breathing and genital 

 organs, the spinnerets, and 

 anal aperture upon its 



Fig. 175. — Spider profiles. 1, Poltys ideae ; 

 2, Phoroncidia 7-acitleata; 3, Ariamnes 

 Jlagdlum ; 4, Stegosoma testudo ; 5, For- 

 'inioinoides h'asiliana. 



TetraUemma medioculatmn, much 

 enlarged. A, Posterior view ; B, profile, 

 showing the scuta. (After Cambridge.) 



the 

 tubercle. 



The breathing organs are, 

 as will be explained later, of yig. 176. 

 two kinds, lung-books and 

 tracheae. The great majority 

 of Spiders possess only two lung-books, and their transverse, slit- 

 like openings (" stigmata " or " spiracles ") may be seen on either 

 side of the anterior part of the abdomen. Where, as in the 

 Theraphosae, there are four lung-books, the second pair open by 

 similar slits a short distance behind the first. According to 

 Bertkau, pulmonary sacs are entirely lacking in the genus Nops. 



The tracheae generally debouch by a single median stigma 

 towards the posterior end of the abdomen, just in front of the 

 spinnerets. This opening clearly results from the fusion of two 

 stigmata, which in some species retain their paired arrangement. 



On a level with the openings of the anterior lung-books or 

 pulmonary sacs there is usually observable a slight transverse 



