\'ol. XXvi] BNTOMOLDGICAL NEWS 39I 



disappearance in the serial sections, is but xxu 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 pigwunt in their own epithelial layers. This 

 pigment b evident both in entire prefnrations and dissections mounted 

 in balaam and alao hi the ac rt Jom from blocks of material embedded 

 in paraffn, mud greatly increaacs die eaae 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 co pacct i ve tsasne envelopes of the frontal 

 ganglion, of the brain and of at IcMt some of the i^nglia 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 functkm. His statements that spadtdn "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 Urval tracheal gills and in those of Aeschmaf\ (p. 69) are 

 not entirely in accord with conditions here described for Tkaumtaio- 

 ntura. 



Transverse sections of the hind end of the abdo m en and of mtdian 

 ond iattfi emmiml giUs show that the tracheae, stippljring 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 sectioiia of the gills nor entire moants of these organs in- 

 dicate any rich deve l opment of tracheae wtdiin Uiem. The thickness 

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

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

 larva no. a, varies from jot2 to .016 mm., and .004 to .016 mm. respec- 

 tively. There is much less bUckish 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 epithelitmi of the gills themselves, but is 

 not uniformly distributed therein. 



The same sections of the median caudal g}\\ show the presence of 

 two median blood s^es, one dorso-central, the other vcntro-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 ao. of our series 



of larva no. 7. 



