250 COMPARATIVE ANATOMY. 



carapace, of which they form outgrowths, as, for example, the 

 setas in certain parts of the body of the Crustacea, the hairs of the 

 Spiders, Caterpillars, etc. ; sometimes they are more developed and 

 only loosely attached to the body, like the scales of Butterflies, which 

 are also present in other divisions, e. g. in the Thysanura. In any 

 case the chitinous investment of the process is continuous with the 

 rest of the integument. In movable appendages of this kind the 

 chitinous layer is softer at the point of attachment, while the cuticle 

 does not change in character on the stiff processes. Integumentary 

 structures, such as denticles and ridges, are also converted into 

 organs of voice in many Insecta (Grasshopper, Cicadas). 



There are glandular organs in the integument, which are 

 derived from modifications of the epidermal layer. They are not so 

 common in the Crustacea as in the Insecta. The secreting portion 

 of the gland consists of one cell merely, or of a few ; and its duct 

 is principally formed by the pore-canals of the cuticular layer 

 (cf. Fig. 7, p. 23). 



The dermal glands are largely developed in certain parts of the 

 body in those Insects that form wax. In the Aphides, and still 

 more in some Hymenoptera, groups of dermal glands are converted 

 into wax-secreting organs. The spinning glands of Spiders are 

 further differentiations of dermal glands. Glands which lie in the 

 abdomen and open by several pairs of papillas placed behind the 

 anus (spinning papillae) produce a secretion, which hardens into a 

 chitinous filament when exposed to the air, and so forms the thread 

 of the Spider's web. An apparatus, which is only functionally similar 

 to this is found in Peripatus. Two groups of ramified tubes pass 

 into an excretory duct on either side ; this duct, which is locally 

 widened out, opens at the base of the second post-oral appendage. 

 The secretion is of a sticky character and rapidly hardens. Morpho- 

 logically, these organs appear to lead to those which are found in 

 the larvas of many Insecta, which, therefore, may be regarded as 

 having had a common origin with them. In the larvae of the Lepi- 

 doptera, and many Coleoptera and Hymenoptera, there is a pair of 

 long and frequently coiled glandular tubes placed beside the euteron, 

 the fine ducts of which have a common opening on the labium. 

 Their secretion forms the silk-threads of the web of these larvas. 

 These " spinning vessels " are most highly developed before they 

 pass into the quiescent pupa stage (Serictarias) ; after the web is 

 made they become atrophied. 



Lastly, other glands seem from their secretion to be poison 

 glands, e.g. those which open on the claws of the spiders, and on 

 the caudal spine of the Scorpion; these add to the variety of 

 differentiations which the glandular system of the integument may 

 undergo. 



