502 INVERTEBRATE MORPHOLOGY. 



1. SUBCLASS APTERYGOTA. 



Tlie members of this subclass are all small and do not 

 possess wings, the absence of these structures being a primi- 

 tive feature and not due to degeneration resulting from para- 

 sitism or other causes. In some forms rudiments of abdomi- 

 nal limbs are present in the adults, and there is no meta- 

 morphosis in the post embryonic development (primary 

 ainetabolisin). 



1. Order Thysanura. 



The Thysanura or Bristle-tails possess ten abdominal 

 segments, the terminal one bearing two- or three-jointed hair- 

 like processes, whence the name applied to the order. The 



body in some forms (Lepisma) is covered 

 with scalelike hairs giving it a silvery-gray 

 appearance, but in other cases these are 

 wanting. The antennae vary in length, but 

 are always simple cylindrical structures, 

 the terminal joint in some forms (Campoded) 

 bearing a peculiar bilobed structure sup- 

 posed to be sensory, and the mouth-parts 

 are adapted for biting purposes and are 

 usually well developed. The first abdom- 

 inal segment in some forms bears a pair of 

 indistinctly-jointed appendages, probably 

 rudimentary limbs, and a number of the 

 succeeding segments in Campodea bear spur- 

 like processes, also supposed to be limbs 

 and recalling the spurs of Scolopend.rella, 

 especially as protrusible glandular struc- 

 tures, comparable perhaps to crural glands, 

 FIG. zw.-Campodea occm> in assoc i at i O n with them in some 



staphyhnus (after 



LUBBOCK from Hux- lOriHS. 



LEY) The nervous system shows but little 



concentration, eight abdominal ganglia occurring in Lepisma, 

 and eyes are usually present, being in some cases compound. 

 The stigmata vary in number, being usually ten, though in 

 Campodea they are reduced to three, and the tracheae in this 



