Vol. XXvi] ENTOMOLOGICAL NEWS 



disappearance in the serial sections, is but .002 mm.* The manner of 

 termination has not been ascertained. 



The great majority of the tracheae in the abdomen, of all sizes from 

 the great longitudinal trunks to the infra-epithelial tracheoles, possess 

 an abundance of blackish pigment in their own epithelial layers. This 

 pigment is evident both in entire preparations and dissections mounted 

 in balsam and also in the sections from blocks of material embedded 

 in paraffin, and greatly increases the ease with which these vessels may 

 be traced. It presents the same appearance as that of the pigment in 

 the "three, longitudinal, thinner pigmented areas" of the rectum, de- 

 scribed above, and in other parts of the body, such as the epithelial 

 layer of the pharynx, the connective tissue envelopes of the frontal 

 ganglion, of the brain and of at least some of the ganglia of the vent- 

 ral nerve cord. 



This pigment is perhaps the same as that for which Purser (1915, 

 p. 67) has proposed the name "spadicin" and which he suggests may 

 have a respiratory function. His statements that spadicin "only ap- 

 pears in the respiratory organs of true aquatic insects" (p. 68) and 

 "is not situated in the tracheal epithelium but in the hypodermis" [of 

 Agrionid larval tracheal gills and in those of Aeschna?} (p. 69) are 

 not entirely in accord with conditions here described for Thaumato- 

 neura. 



Transverse sections of the hind end of the abdomen and of median 

 and lateral caudal gills show that the tracheae, supplying the latter 

 organs do not enter them as single, relatively large trunks, but that 

 each trachea divides and redivides into a number (four or more) of 

 subequal tracheae which collectively pass into the base of the gill. 

 Neither the sections of the gills nor entire mounts of these organs in- 

 dicate any rich development of tracheae within them. The thickness 

 of the chitinous cuticle of the proximal joint, as measured in cross- 

 sections of the median caudal gill and of one lateral caudal gill of 

 larva no. 2, varies from .012 to .016 mm., and .004 to .016 mm. respec- 

 tively. There is much less blackish pigment in the epithelial layer of the 

 tracheae of these gills than in that of the tracheae of the abdomen. 

 Pigment is present in the epithelium of the gills themselves, but is 

 not uniformly distributed therein. 



The same sections of the median caudal gill show the presence of 

 two median blood spaces, one dorso-central, the other ventro-central, 

 situated respectively ventrad and dorsad to other blood spaces which 

 largely occupy the mid-dorsal and mid-ventral longitudinal carinae of 

 the proximal joint. These two central blood spaces have thin but dis- 

 tinct walls, contain plasma and corpuscles in their lumens and lie in the 

 midst of a reticulated or spongy tissue ; their transverse diameter, near 



*E. g., in the dorsal fold, slide 5, row 3, section 20, of our series 

 of larva no. 7. 



