114 The Life-History of Insects 



tubes capable of absorbing the dissolved air. A very 

 beautiful adaptation seen in many aquatic larvae — that 

 of the Gnat for instance — is a valve consisting of 

 several leaf-like plates, which close a spiracle at the 

 tail end of the insect (fig. 72 a). The points of these 

 valve-plates are capable of piercing the surface-film 

 of the water, and the plates then expand so as to form, 

 in conjunction with the depressed film, a cup contain- 

 ing air and communicating freely with the insect's 

 body as it hangs from the surface, head downwards. 

 Should such a larva have occasion to dive, sufficient 



Fig. 73. — a. Cast-off first nymphal skin or Bed-bug (Ciiiux lectularitis) ; b. 

 second instar after emergence from a. \ c. the same after a meal. Magnified 

 30 times. From Marlatt, Bull. 4 (n.s.) Div. Ent. U.S. Dept. Agr. 



air can be stored in the body to allow submergence 

 for a considerable time (170). 



Moulting'. — It is evident that the plan on which 

 insects, as well as all arthropods, are built up — a hard 

 outer skeleton surrounding the muscles and soft 

 organs — necessitates a casting of the skin at successive 

 periods as the creature grows, since the hard, un- 

 yielding texture of the chitin forbids expansion, either 

 of itself, or, beyond certain limits, of the contained 



