40 THE CLASS OF INSECTS. 



full-grown caterpillar ; we have counted about sixty a minute 

 in the recently hatched larva of Dif)lax. During excitement, 

 the number of pulsations increases in rapidity. Newport found 

 the pulsations in a bee, Antliophora, when quiet, to be eighty a 

 minute ; but when "the insects were quite lively, and had been 

 exposed to the sun for an hour or two, the number of pulsa- 

 tions amounted to one hundred and forty." 



He found that the number of pulsations decreased after each 

 moult of the larva of Sphinx ligustri, but increased in force; 

 when it was full grown and had ceased feeding it was thirty. 

 "After it had passed into the pupa state the number fell to 

 twent3'-two, and afterwards to ten or twelve, and, during the 

 period of hibernation, it almost entirely ceases ; but in the per- 

 fect insect it rose from forty-one to fift3 T , and when excited by 

 flight around the room it was from one hundred and ten to one 

 hundred and thirty-nine." 



ORGANS OF RESPIRATION. All insects breathe air, or, when 

 they live in the water, respire, by means of branchiae, the 

 air mixed mechanically with water. Respiration is carried on 

 by an intricate system of tubes (pul- 

 monary tracheae) which open by pores 

 (spiracles or stigmata) in the sides of 

 the body ; or, as in aquatic insects, by 

 branchiae, or gill-like flattened expan- 

 sions of the body-wall penetrated by 

 tracheae (branchial tracheae). 



There are normally eleven spiracles, 

 or breathing-holes (Fig. 48), on each side 

 of the body ; each consisting of an oval 

 horny ring situated in the peritreme 

 Fl s- 48 - and closed by a valve, which guards 



the orifice (Fig. 49). Within this valve is a chamber closed 

 within by another valve which covers the entrance into the 

 tracheae. The air-tube itself (Fig. 50) consists of "an external 



FIG. 48. Larva of the Humble-bee just beginning to change to a pupa, showing 

 eleven pairs of stigmata. In the adult bee, only the third pair is apparent, the 

 remaining pairs being concealed from view, or in part aborted. In most insects 

 there are usually only nine pairs of stigmata. Original. 



