-362 



THE AGRICULTURAL NEWS. 



November 13, 1909. 



INSECT NOTES. 



NATURAL HISTORY OP INSECTS. 

 Part II. Structure and Growth. 



As ha.s been already stated, inscct.s are aiiinials wliicli, in the 

 adult condition, liave never more than .six leg.s,and generally, 

 two pairs of wings (which may be wanting but are never more), 

 and a body made up of three distinct parts, head, thorax and 



Fig. 43. Cockroach. (U. S. Department 

 abdomen. The legs are 

 attached to the thorax, 

 one pair to each segment; 

 khe fore and hind wing.s 

 are attached to the second 

 and third segments, res- 

 pectively. The head and 

 abdomen have no organ.s 

 of locomotion. To the 

 head are attached the 

 mouth parts, and the 

 ^antennae wliicli are sen- 

 sory organs. 'J'he abdomen 

 also sometimeshas sensory 

 organs. Fig. 13 shows 

 the cockroach and the 

 attachments of wings and 

 leg.s. 



In the case of most 

 insects, tlie adult deposits 

 eggs from which the young 

 are hatched. There are 

 instances, however, among 

 thes3 animals, in which 

 the young arc already 

 hatched from the egg at the time w 



riculturt 



Fig, 44. I!ean Lkak 1!ollei:. 



egg, it goes through changes and development.s before reaching 

 the fvdly developed or adult condition. This change and 

 development is called the metamorphosis. JIetamorrjhosi.s 

 may be either complete or incomplete. This does not mean 

 that development stops short of the [jcrfect insect, in the case 

 of insects which have incomplete metamorphosis, but rather 

 exjjresses a general dilierence between these two kinds of 

 development. 



Insects which have an incomplete metamorphosis 

 are something like the adult when they are first 

 hatched from the egg. (Fig. 43, young (c) and adult 

 (ii and //) cockroaches.) They are, of course, much 

 smaller, and without wings, but in general form they give 

 an idea of what the}" will be like when they are full-grown. 

 Insects which have a complete metamorpliosis are verv dif- 

 ferent, when first hatched, . from the adult form into which 

 they finally develop. Such insects have four distinct stages, 

 or period.'', in their lives. The.se are, the egg, the larva, the 

 (lupa and the imago. The egg is the first stage in the 

 (Icvelopment of a generation, and the larva is the next. The 

 larvae of the butterflies and moths are called caterpillars; of 

 beetles, grubs; and of files, maggots. This is the portion of 

 an insect's life when all growth in size takes place. The 

 pupa is the state during which the change takes place, in 

 which the larva is transformed into the final and adult 

 cciudition of the insect's life; the wings are formed and the 

 reproductive organs become comiilete. The pupa of a butter- 

 lly is called a chrysalis; that of a moth is often enclosed in 

 a cocoon. Fig. 44 shows the bean leaf-roller {Ei(damus 

 pi-oteus) with larva (/y and c), pupa (rf), and adult (a). 

 In the case of insects which have an incomplete metamor- 

 pho.si.s, there is not the same distinction between the larva and 



pupa, as in the case of 

 those having a complete 

 one. There is no ipiiescent 

 stage during which the 

 iu-sect transform,';, as in the 

 chry.salis of the butterfly. 

 y ,',/^ The growth of insects i.s 



'U/lji!^ accompanied by a series 



of moults. The chitinous 

 exoskeleton, once formed 

 and hardened, is not 

 capable of growth, and it 

 'i,;;-:'is natural that it .should 



^■'"' ^'^ ' ' •■--"' ' -, ;---k, . ; ■K.,^ be shed at intervals, when 



the limbs and organs have 

 increased in size as much 

 as they can. liefore the old 

 skin is cast ofl", there i.s 

 formed under it another, 

 which is soft, pliant ami 

 elastic. As soon as the old 

 one is shed, the ncw^ skin 

 is stretched out by the 

 pressure from within, and 

 it then hardens. The 



(U. S. Department of Agriculture.) 



on they are born, 'i'hat is to exoskeleton, as it now becomes, is fixed, and docs not grow 



»ay, they are active and free-moving, having accomplished one any more. The .size of the insect is stationary until the next 



stage of their existence w^ithin the body of the parent. moull, except that the fiexiblc connective tissues between the 



After the birth of an insect, or the hatching from the .scguicnls are capable of strct<-liing a little. 



