HEXAPODA 



27 



How Insects Grow 



It has already been pointed out that a small fly does not grow into a 

 large fly. No insect grows after it becomes an imago. All growth of an 

 insect takes place in the young stages and larval and nymphal stages; 

 and growth in these immature stages is accompanied by a shedding or 

 molting of the outer skin. 



Molting. — The outer skin of a nymph or of a larva consists in large 

 part of a substance known as chitin. In the young insect this outer 

 skin soon becomes so firm and hard that it will not stretch enough to 



Fig. 53. ■ — Leg of May-beetle. (After Straus-Durckhenn.) 



allow for the growth of the insect. The result is that 

 from time to time the skin of the young insect becomes 

 too small for it and must be shed. But before this is 

 done a new skin is formed beneath the old one; then the 

 old skin bursts open, and the insect crawls forth, clothed 

 in a soft skin, which stretches to accommodate the in- 

 creased size of the individual. Very soon, however, this 

 new skin becomes hardened, and after a time it in turn 

 must be shed. This shedding of the skin is termed molt- 

 ing, and the cast skin is sometimes referred to as the 

 exuvicz. Insects differ greatly as to the number of times 

 they molt: many species molt only four or five times, 

 known to molt more than twenty times. Figure 52 



Fig. 52. — Exuviae 

 nymph of dragonfly. 



while others are 



represents the cast skin of a dragonfly clinging to a reed. 



The External Anatomy of Insects 



The subject of insect anatomy is separated into two divisions: one, 



treating of the structure of the 

 body- wall or skeleton ; the other, 

 of the internal organs. The for- 

 mer is termed external anatomy; 

 the latter, internal anatomy. 



In our own bodies we find a 

 central framework or skeleton, 

 about which are arranged the 

 muscles, blood-vessels, nerves, 

 and other organs. But insects 

 are constructed on an entirely 

 different plan; with them the 

 supporting skeleton is outside, 

 and the muscles, nerves, and 

 — a section of the body-waii of an insect: other organs are within this 



/;, hypodermis; bm, basement membrane; 1 1 / rrvi tj?c u 



d, dermis; tr, tdchogen; s, seta. skeleton. The difference can be 



Fig. 54- 

 <-, cuticula; 

 e, epidermis. 



