CONDITIONS OF LARVAL INSECT LIFE. 9 



insects receive a plentiful supply of tracheal branches at an early stage, and it is 

 evident that these are much in excess of any aeration the wings are likely to 

 require. The tracheal development seems to be a persistence from an earlier 

 more active condition, when the larval wings may have played a part in assisting 

 the respiration. These considerations, and the presence of spiracle-like structures 

 in the interstitial neuration of the adult wings of many Palaeodictyoptera, lend 

 support to the inference that the wings functioned as organs of respiration. These 

 spiracle-like structures are usually oval or rounded in outline, and thickened. In 

 some instances they show a series of raised lines radiating from the thickened 

 edge into the surrounding areas, as if they had been muscular strands and capable 

 of expansion and contraction. 



Are these structures the atrophied remains of spiracles once functional, and 

 fitting the larva for a more or less aquatic existence ? During a recent 

 visit to this country Dr. Tillyard has suggested to me that they are 

 rudiments of sensory organs which ma} T have been scent-glands. Scent- 

 glands are known to occur on the wings of many insects, as, for example, the 

 Green-veined White (Pieris napx), the Small White {Pieris rapae) and others, 

 and their appearance is certainly a strong argument in favour of the view. Scent- 

 glands are, however, in all probability but specialised developments of previously 

 existing structures, and it is possible that the glandular-like organs to which I 

 give the name of "pseudo-spiracles," and Handlirsch the name of "pterostigmata," 

 are an earlier development connected with the " tracheoles " of Comstock, or that 

 primary tracheolation to which Tillyard has given the name of " archyodictyon." 

 Tillyard does not accept the view that they had ever any connection with 

 respiration. 



The almost total absence of structures which can be accepted as functional 

 gills in these fossil insect-larvae may be accounted for by the perishable nature of 

 such organs. Before dismissing the question of the respiratory function in its 

 relation to the conditions under which larval life was passed, it is desirable to draw 

 attention to the larval Blattoid, Leptoblattinct exilis, Woodw. In this insect the 

 abdominal segments have the dorsal hinder margin extended into broad lamellar 

 expansions so filmy in texture that they may have served as organs of aeration. 

 The lamellar expansions were longer in life than they now are, the hinder borders 

 showing an irregular torn edge. Their extreme thinness would permit of a read} r 

 osmotic-like action, especially in damp vegetation, or in an aquatic or semi-aquatic 

 habitat. Scudder, Handlirsch, Lameere and others are all agreed in the belief 

 that the Blattoids freouented decavino- vegetation in or near water, and under these 

 conditions the presence of organs of aeration similar in character to the abdominal 

 expansions of Leptoblattina exilis would be of the greatest value and offer no 

 difficulties to the habit of life. No similar structures are known in any other 

 larval Blattoid, so that the view cannot be pressed. 

 2 



