MODE OF SPINNING COCOONS 621 



the integument of the semipupa. To this cocoon-like covering of 

 the coarctate pupa we have restricted the term puparium, originally 

 used by Kirby and Spence to designate the pupa. The puparium 

 is usually cylindrical or barrel-shaped, rounded at each end. 



In the Diptera eydorhapha, or common house and flesh flies, etc., 

 the puparium remains in vital connection, by means of four tracheae, 

 with the enclosed pupa, which escapes from the case through a 

 curved seam or lid at the anterior end 

 and not by a slit in the back, as do the 

 orthoraphous families, represented by the 

 horse-fly (Tabanidee, Asilidse, Fig. 570), 

 etc., where in some cases the obtected pupa 

 remains within the loose envelope formed 

 by the old larval skin, which Brauer calls 

 a false puparium. The dry, hard puparium 

 is burst open at the cephalic end when 

 the fly emerges, by means of the frontal exit of fly -After ciark, from 



J Osborn, Bull. 5, l)iv. Ent. U. S. 



vesicle, which is distended with fluid (Fig. Dept. Agr. 

 571). 



The exact mode of spinning the cocoon by caterpillars has been 

 carefully observed by L. Trouvelot in the case of the polyphemus 

 silkworm. 



" When fully grown, the worm, which has been devouring the leaves so vora- 

 ciously, becomes restless and crawls about the branches in search of a suitable 

 place to build up its cocoon ; before this it is motionless for some time, holding 

 on to the twig with its front legs, while the two hind pair are detached ; in this 

 position it remains for some time, evacuating the contents of the alimentary canal 

 until finally a gelatinous, transparent, very caustic fluid, looking like albumen, 

 or the white of an egg, is ejected ; this is a preparation for the long catalepsy that 

 the worm is about to fall into. It now feels with its head in all directions, to 

 discover any leaves to which to attach the fibres that are to give form to the 

 cocoon. If it finds the place suitable, it begins to wind a layer of silk around a 

 twig, then a film.- is attached to a leaf near by, and by many times doubling this 

 fibre and making it shorter every time, the leaf is made to approach the twig at 

 the distance necessary to build the cocoon ; two or three leaves are disposed like 

 this one, and then fibres are spread between them in all directions, and soon 

 the ovoid form of the cocoon distinctly appears. This seems to be the most 

 difficult feat for the worm to accomplish, as after this the work is simply me- 

 chanical, the cocoon being made of regular layers of silk united by a gummy 

 substance. The silk is distributed in zigzag lines of about one-eighth of an inch 

 long. When the cocoon is made, the worm will have moved his head to and 

 fro, in order to distribute the silk, about 254,000 times. 



" After about half a day's work, the cocoon is so far completed that the worm 

 can hardly be distinguished through the fine texture of the wall ; then a gummy 

 resinous substance, sometimes of a light- brown color, is spread all over the inside 

 of the cocoon. The larva continues to work for four or five days, hardly taking 



