426 LECTURE XVIII. 



the suspension of all the ordinary actions of life. The larval skin is 

 thrust off by the new integument of the new organs, and is converted 

 into an opaque brown case ; the enclosed insect shrinks partly by the 

 loss of exhaled fluids, partly by the condensation of its former soft 

 tissues into the new and firm substances constituting the legs and 

 wings. A large and distinct head is now developed, with eyes, an- 

 tennae, and instrumenta cibaria ; all which processes are carried on 

 in the quiescent concealment of the opaque and dark exuvium, like 

 the analogous processes in the egg of the oviparous, and within 

 the womb of the pupiparous, insect. The active carnivorous ver- 

 milarve returns, in fact, a second time to the state of an ovum, 

 when it becomes the coarctate pupe ; and the perfect insect, split- 

 ting its cerement, issues forth as by a second birth. 



The larvae of the gnats ( Culex) and crane-flies ( Tipulce) have a 

 distinct corneous head with jaws ; the former have a plumose anal 

 coronet, by which they sustain themselves at the surface of the 

 water ; the orifices of the tracheae are placed in the middle of this 

 coronet. A pair of tracheal tubes extend through the long, slender, 

 and extensile anal canal of the aquatic grub of the Musca {Eristalis) 

 tenax. By this mechanism, which is analogous to the tube of the 

 diving-bell, the rat-tailed larva can derive its requisite supply of air 

 from the surface while groping for food in the mud at the bottom of 

 the pool. 



Insects differ much in their dependence on the stimulus of heat 

 for different generative processes, the Diptera being the hardiest. 

 Some gnats come forth during the depth of winter and a continuance 

 of frost, if merely the sun's rays for a while produce a slight rise of 

 temperature. And amongst the Perlidce the Canadian Capina ver- 

 nalis comes forth at the end of winter, when the thick ice begins to 

 crack, and changes from nymph to imago in the crevices, leaving its 

 slough there, even when the temperature of the air has again sunk 

 to freezing. Brachyptera glacialis even pairs in the crevices of the 

 decaying ice. 



The economy of the Hymenoptera^ and the various circumstances 

 attending the development of the apodal larvje, form the subjects of a 

 long chapter in the History of Insects. 



I must be governed in the unavoidably limited selection from this 

 rich storehouse of interesting facts by the specimens which Hunter 

 has left for our instruction. In No. 3104, we have a portion of the 

 nest of a- social hymenopterous insect of the wasp tribe {Polistes 

 major), showing the larvae and their cells in every stage of growth ; 

 the smallest larvae and the shallowest cells are at the lower margins 

 of the pendent nest ; and observe how, in these beginnings of cells. 



