ANATOMY AM) I'll YSK >I.O(i Y 



119 



i: i e adaptation- are often supplemented hy waving i 



the trills, as in May tly nymphs, and by frequent movement- of the 



! from one place to another. 



I : N ially notewort hy are t he rtrliil tnicht'til gills of odonate nymph-. 

 In the-e Insects the lining of the rectum form- numerou- papillae OF 

 lamella-, which contain a profusion of delicate t ra- 

 dical branches; t hc.-e arc bathed by water drawn 

 into the rectum and then expelled, at rathep irregu- 

 lar intervals. A similar rectal respiration occurs 

 also in ephemerid nymphs and mosquito larva-. Fi.. 173- Caudal 



A few forms, chiefly IVrlidae, are exceptional in nymph, enlarge'd. 56 

 retaining tracheal <jills in the adult stage; in some 

 imagines they are merely vestiges of the nymphal gills, but in others, 

 such as Ptcronarcys (Fig. 19), which habitually dips into the water 

 and rests in moist situations, the gills probably supplement the spiracles. 

 Further details on the respiration of aquatic insects 

 are given in Chapter IV. 



Blood-gills. In a few aquatic larvae, Simulium 

 and Chironomus for example, there are thin tubular 

 evaginations of the integument known as blood-gills, 

 into which the blood flows. In trichopterous larvae 

 (caddis-worms) the blood-gills are eversible. Some 

 authors regard the ventral eversible sacs of Scolo- 

 pendrella and Thysanura, as well as the vesicles of 

 the ventral tube of Collembola, as blood-gills. 



Spiracles. The paired external openings of the 

 tracheae, termed spiracles or stigmata, occur on the 

 sides of the thorax and abdomen; there being not 

 more than one pair to a segment, though not all 

 segments bear them. As a rule, there are two tho- 

 racic and eight abdominal pairs; these belonging to 

 FIG. 174. Cater- the mesothorax, metathorax, and first eight abdom- 

 olscuraiis. to^h'owTra- ma ^ segments, respectively. Adult insects do not 

 cheai gills. Length, 15 have more than ten pairs, with the exception of 



mm. After HART. . 



Japyx (see page 60, footnote). 



The spiracles, variable in position, are situated usually between two 

 segments of the body; but often at the anterior borders of the seg- 

 ments to which they belong; though they may occur farther back on 

 the segr- ?nts. 



In most embryo insects there are eleven pairs of spiracles three 



