THE WAX-GLANDS 



363 



They are leaf-like in the first nymphal stages of Trioza rhamni, but 

 in following stages become narrow and form a row around the entire 

 periphery of the body. 



The waxen dorsal shield which protects the body of bark-lice 

 (Coccidse) is a similar product. 



Witlaczil has described the way it is formed in Aspidiotus and Leucaspis. 

 The freshly hatched nymph shows no signs of a waxy secretion. But eventually 

 waxen threads arise first on the hinder and anterior end of the body, and then 

 over the whole surface. These threads interlace into a sort of felting and thus 



FIG. 854. Young nymph and developing scale of Aspidiotus perniciosus : a, ventral view of 

 nymph, showing- sucking- beak with set* separated, with enlarged tarsal claw at right ; b, dorsal view 

 of <ame, somewhat contracted, with the first waxy filaments appearing ; c, dorsal and lateral views 

 of same, still more contracted, illustrating further development of wax secretion ; il, later stage of 

 same, dorsal and lateral views, showing matting of wax secretions and first form of young scale; all 

 greatly enlarged. After Howard and Marlatt, Bull. 3, N. s., Div. Ent., U. S. Dept. of Agr. 



form the shield, which is usually much larger than the body and lies closely 

 upon it. The shield is formed after the first moult. It is noteworthy that these 

 threads are matted together to form as thick a tissue as that of the shield itself. 

 The shield is whitish or gray and rather thin. On the thinnest part of the edge 

 the single threads may be drawn out. The growth of the shield advances with 

 the increase in size of the nymph around the entire edge, but is greatest behind. 

 The first two larval skins are retained on the back under the shield. Also a 

 very thin waxen pellicle remains on the resting place of the insect when it is 

 raised. The wax-glands open in the pitted fields, and appear as clear brownish 

 cells which are distinguished from the ordinary hypodermis cells by their greater 

 size. (Witlaczil. Compare also Fig. 354.) 



