38 ARKIV FÖR ZOOLOGI. BAND 8. NIO 9. 



It may suffice to mention that it enables them make a cocoon 

 when pupating, hybernating or before undergoing an ecdysis, 

 or to suspend the pupa; or twist and fold leaves together 

 in order to protect themselves; or enables them to slide 

 gently down from a tree or plant when alarmed; or fi- 

 nally to walk on any surface by constructing a kind of 

 ladder on which their feet can get a secure hold. The last 

 function is probably not by any means the least important 

 and if we observe above all the younger stages of many 

 larvae, we find that they attach a thread in a zic-zac manner 

 in front of themselves to the object on which they are walking 

 and gradually moving forward. In fig. 57 is delineated the 

 thread attached by the larva of Coleophora laricella to a 

 glass-slide. To this rule nearly all tissue-feeders amongst 

 the life-long mining larvae form an exception, their spinning 

 faculty being brought into action only when constructing 

 a cocoon either for hybernating or pupating purposes. The 

 only larva I know of, which makes use of its spinning glands 

 in the mines, is that of Tischeria ekebladhella, the others, 

 such as Eriocrania, Nepticula, Cemiostoma, Lyonetia and Ela- 

 chista do not spin when mining. 



It is evident that this habit forms a profound difference 

 between these larvae and the external leaf-eaters, and has 

 been acquired contemporaneously with their mining habits 

 from want of use of the spinning faculty, as it is necessary 

 neither for their locomotion nor protection. 



When an organ becomes useless it generally either becomes 

 reduced or even completely atrophied; or it survives by chang- 

 ing its function, and being modified. 



Amongst the tissue-feeders described above we find both 

 these alternatives realized. 



Thus in the series Eriocrania — Lyonetia — Cemiostoma la- 

 ournella — C. scitella we notice that the spinneret, and partly 

 also the labial palpi, decrease in size, and at the same time 

 move backwards, so that in the young larva of the last men- 

 tioned species we can only find scarcely discernible traces 

 in the form of a pair of minute hairs. 



This retrogressive development is accompanied by the 

 progressive development of the hypopharynx, so as to form 

 a large, tongue-shaped organ which constitutes the floor or 

 under lip of the mouth-opening, flanked by the maxillse. 



