120 The Life-History of Insects 



larva having passed its life underground or buried 

 itself before pupation. Sometimes foreign matters 



are worked into the co- 

 coon, as in the case of 

 timber-living larvae, 

 which strengthen their 

 cocoons with chips of 

 wood ; or such material 

 may form the greater part 

 of the cocoon, as in the 

 case of many caterpillars 

 which simply roll leaves 

 and fasten them together 

 with a few threads (fig. 

 77)- The cocoon when 

 completed may contain 

 ^ ^ so much silk, so densely 



Fig. 78. — Cocoon surrounding pupa of r i. j 



Silkworm Moth {Bomhyx jnori). From WOVeU aS tO torm a hard 



Riley, Bull. 9 Div.Ent. U.S. Dept.Agr. shell-like case, as that of 

 an Eggar moth. Many caterpillars are known to 

 secrete acid from a gland opening beneath the pro- 

 thorax, which renders the 

 silk very firm (78 b) ; or the 

 cocoon may be a loose mesh- 

 work within which the pupa 

 lies, as that of certain Leaf- 

 beetles. In Butterflies it is 

 reduced to a pad of silk, by 

 which the pupa is suspended, 

 with the addition, in some 

 cases, of a single thread en- 

 girdling the pupa and affix- 

 ing it to a support. In the 

 vast majority of two-winged 

 Flies, the last larval skin, instead of being worked oft 

 by the usual process, continues to surround the pupa 



Fig. 79. — Puparium of Warble-fly 

 {Hypoderma bovis). a. side 

 view ; b. ventral view showing 

 opening for escape of fly left by 

 rupture of lid c. Natural size. 

 From Osborne (after Clark), 

 Bui!. 5 (n.s.) Div. Ent. U.S. 

 Dept. Agr. 



