Characteristics of Insecta. cix 



abdomen, but there are not wanting more or less obscure indications 

 of the presence of three segments between the generative outlet and 

 the terminally-j)laced anus, which may be considered as representing 

 the post-abdomen. The aljdomen proper never carries any articu- 

 lated appendages in adult insects, with the single exception of the 

 Coleopterous Sjnracktha Euri/medusa, in which the third, fourth, and 

 fifth abdominal segments each carry a pair of biarticulated appen-. 

 dages. The post-abdominal segments however may carry segmented 

 appendages both in the adult and in the larval condition, but 

 these organs, though they may attain a considerable development, 

 especially in Orthoptera {Chloeon dlmicliatuni), do not appear to 

 possess locomotor functions in adult insects. The motor organs are 

 mainly localized in the thoracic, the vegetative in the post-thoracic 

 regions. None of the thoracic segments are ever, except in certain 

 Coccina, fused with the cephalic, nor are any thoracic appendages 

 ever modified so as to serve as manducatory organs. The greater 

 relative size of the eyes gives as distinctive a character to the head 

 in this Class of Arthropoda as its freedom from fusion with the 

 thorax. The mandible has never even a rudiment of a palp ; and 

 the second pair of maxillae are always more or less completely 

 soldered together so as to form a functional lower lip, the ' labium ' 

 of entomologists. The digestive canal is never aproctous except in 

 the larvae of most Hymenoptera, of the parasitic Diptera, and of 

 Myrmeleo. In these larvae the renal organs open into what is 

 subsequently by moulting brought into communication with the 

 blind end of the digestive tube, and so converted into a ' rectum;' 

 the entire apparatus previously to this change bearing a striking 

 resemblance to the so-called ' water- vascular ' or excretory system 

 of certain Vermes. The digestive tract presents more numerous 

 and more distinctly distinguishable divisions than in other Arthro- 

 poda ; and it is often, at least in adult insects, arranged in convolu- 

 tions, and is thus longer relatively to the body than in other mem- 

 bers of the Sub-kingdom. The salivary glands may be large and 

 racemose as in Orthoptera, or very small and tubular as in the adult 

 Lepidoptera. The liver appears to be represented by certain coeca 

 which are set round the commencement of the digestive tract in 

 varying numbers in the Ortlioptcra and Hemiptera, the great 

 development of the respiratory tracheal system appearing to com- 

 pensate for this rudimentary or aborted condition of the hepatic 



