Introduction to Anvnal Morphology. 397 



sharp differentiation of the thorax, assume its adult 

 form. Insects which only undergo changes like these 

 are called ametabolous. Other insects, while under- 

 going the growth of wings, &c., lose embryonic 

 organs, and are called hemimetabolic. Most, how- 

 ever, differ very much in their larval and perfect states, 

 and the former passes through a quiescent or pupa 

 period before it becomes mature ; these are holome- 

 tabolic. The larvae, or caterpillars in general, are 

 annelid-like, nearly homonomous, with a soft skin, 

 and stumpy or no feet ; when present, the three an- 

 terior pair are often discriminable from the others 

 (which are called prolegs*), in shape and colour, and in 

 being several-jointed (often five). They are voracious, 

 with powerful jaws, a large stomach, and a large fatty 

 body ; they grow rapidly, and moult frequently. On 

 attaining their full size, those with labial silk-glands 

 spin a cocoon ; others become quiescent, inclosed in a 

 thickened skin, and cease to eat. This pupa or 

 chrysalis stage lasts until the internal organs attain 

 perfect development. They seem to undergo a nearly 

 perfect histolysis, and from certain areas of formative 

 tissue, called imaginal discs, rearrangement takes 

 place ; then the pupa skin splits along the back, and 

 the imago or perfect adult is produced. The pupa 

 may hQ free when the locomotory organs remain free 

 from the trunk, as in Hymenoptera and Coleoptera, 

 or ohtected when these organs are iaclosed in the 

 pupa skin, and so are covered, as in Lepidoptera, or 

 coarctate, when the whole body is wrapped up, with 

 no trace of limbs, as in Diptera. On becoming free, 



* Prolegs may be seven pair (Tenthredo), or eight (Cimbrcx). 



