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AETHEOPODA 



273 



time the nuclei and cytoplasm of the ectodermal cells of the wing 

 migrate outwards, and the basal portions of these cells contract till 

 they form attenuated fibres attached to the basement membrane ; 

 in this way wide spaces are formed between neighbouring ectoderm 

 cells. 



In the wing rudiment of the larva there are two tracheae arising 

 from the dorsal longitudinal trunk, which grow in along the course 

 of the wing veins. Towards the end of the larval period these 

 tracheae give off abundant branches, the tracheoles, which penetrate 

 into the spaces between the bases of the ectoderm cells, and in each 

 of these cellular outgrowths a delicate coiled tube develops. During 

 the pupal stage these tracheoles 

 seem to be absorbed by the blood, 

 but new tracheoles are developed 

 in the adult wing as evaginations 



ant 



Fig. 218. — Two views of the head of the larva of Dytisais marginalis, m two stages 

 of development. (After Glinther. ) 



A, in aquatic larva. B, in larva about to pupate, ant, antenna ; es, eye-spot of the larva ; eisi, lens of 

 eye-spot carried away by loosening cuticle; mn, mandible; mx^, first maxilla ; ma^, second maxilla,; 

 oc.c, rudiment of compound eye ; ocs, simple eyes ; oc^si, lenses of the simple eyes carried away 

 from thein by the loosening of the cuticle. 



from the tracheae. Besides tracheoles, blood and leucocytes are 

 the only other elements which enter the wings. The muscles 

 moving them are confined to their bases and are attached to the 

 thorax. 



The development of the larval and adult eyes has been most fully 

 worked out by Gunther (1912) in the case of Dytiscus marginalis, a 

 beetle belonging to the family of water-beetles (Dytiscidae). The larva 

 has six ocelli on each side of the head, and in addition a rudimentary 

 eye-spot. The ocelli are arranged in a vertical ellipse, and when 

 examined by sections each is found to constitute a slit-like pit. 

 The cells constituting the base of the pit carry visual rods ; each 

 rod consists of two semicylindrical pieces adherent to each other, 

 and each piece seems to consist of a mass of agglutinated delicate 

 fibrillae. The cells lining the sides bear horizontally directed 



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