318 ARACHNIDA—ARANEAE CHAP, 
That these scuta are sometimes indicative of an obsolete 
seginentation would seem likely from the study of the remarkable 
species, Tetrablemma mediocu- 
Wie latum (Fig. 176), described by 
| Pickard - Cambridge, — from 
ee Ceylon. In addition to large 
Fol dorsal and ventral scuta, the 
: sides and posterior extremity 
A are guarded by smaller scuta, 
the disposition of which is 
well seen in the figure. 
The normal smooth  ab- 
eee domen presents dorsally no 
6 Ze very striking features. In 
Fic. 175.—Spider profiles. 1, Poltys ideae ; species of variegated colora- 
2, Phoronedia 7-aculeata & 3, Ariamnes tion there is very generally 
flagellum ; 4, Stegosoma testudo ; 5, For- i 5 
micinoides brasiliana. 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 
the anal aperture upon its 
- tubercle. 
The breathing organs are, 
as will be explained later, of Fic. 176.— Tetrablenma medioculatum, much 
two Kinds, lung-books and singel Ay Fosters ‘isn 5B of 
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 Jops. 
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 
