Blood-gills 83 



elevated upon a long dorsal stalk, an outgrowtli from tlie 

 eleventh or penultimate segment. The aquatic larva of 

 a Muscid, Ephydra, has two separate tubes, each fringed 

 at its extremity by a circle of setae. In the larvae of 

 Ptychoptera and Bittacomorpha the twelfth segment is 

 very long, slender, and retractile, and the minute spiracles 

 open at its extremity. 



(&) THE BEANCHIAL SYSTEM. 



Insect larvae which live immersed in water often Biood-giiis. 

 develop gills, which are thin, transparent extensions of 

 the body-wall, filled with blood, and employed for 

 respiration. According' as they contain tracheae or not, 

 they may be distinguished as tracheal gills ov Mood-gills^. 

 They have in general little morphological constancy, and 

 vary much in position and number, as well as in minute 

 structure. It is remarkable that functional gills are 

 veiy rarely found in an adult insect, however aquatic 

 its propensities (Packard, 1898, p. 476). 



The larger species of Chironomus-larvae, such as C. 

 dorsalis and C. plumosus, are furnished with two kinds 

 of blood-gills, but tracheal gills are entirely absent. Two 

 pairs of blood-gills are borne upon the lower surface of 

 the last segment but one (fig. i). These are long and 

 flexible, but incapable of independent movement. From 

 the last segment and close to the anus, two pairs of much 

 shorter blood-gills project (fig. i). We find, therefore, 

 two pairs of ventral, and two pairs of anal blood-gills. 

 The hinder end of the body, when the larva is not actually 

 feeding, is often seen to be thrust out from the burrow. 

 When the larva is completely concealed and apparently 

 at rest, it keeps up a vertical undulatory movement of 

 its body within the burrow, which continually renews 



' This distinction, though often convenient, is not strictly .applicable to 

 everj' known case. There are gills whicli are neither tracheal gills nor 

 blood-gills. 



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