30 PROCEEDINGS OF THE ACADEMY OF 



necessity for their use mainly occurs in the interval between the 

 last skin moulting and assumption of the pupa form, they would 

 attain during this period their functional maturity. There may 

 be instances where growth has received a check through adverse 

 climatic influences or otherwise, and the functional activity of 

 these glands has been early attained. If in Acronycta the mature 

 stage had been reached, and the nutriment provided was insuffi- 

 cient to maintain both the life of the larva and the functional 

 activity of the glands, it seems evident that the most vital organs 

 would be sustained at the hazard of those less vital. 



In larvae which do not manufacture cocoons the silk-bearing 

 glands exist, but so reduced in size that they are functionally use- 

 less where great quantities of silk are required. It is true these 

 larvae manufacture silk, but in such exceedingly limited quantities 

 as to be of little or no service for cocoon-making. The Diurnals, 

 as is well known, are able to spin several strands, by which they 

 suspend and engirdle themselves when about to pass into 

 chrysalides. Some moths of the great family Bombycidse make 

 no cocoons, but these are the exceptions rather than the rule. 

 The major part of them are cocoon-builders. While some con- 

 struct the merest apology, others, Cecropia, for example, attain 

 to the highest perfection in the art. The Arctians produce very 

 little silk in comparison with Cecropia ; their silk-bearing glands 

 being necessarily small. The larva of Arctia achaia, Grote, spins 

 a very thin web of a drab color, through which the chrysalis is 

 distinctly seen. The Sphinges would seem to occupy an interme- 

 diate position ; the earthen cells which they build, with their slight 

 lining of silk, being the homologues of cocoons, since they subserve 

 the same essential purpose. Thyreus Abbottii seems to be a real 

 exception. But no. It ordinarily seeks some sheltered locality 

 in which to undergo its critical changes the under surface of a 

 fallen log or a board, where an abundance of decomposing leaves 

 affords the necessary warmth and shelter. It is obvious that the 

 necessity for a cell does not exist, if the foregoing facts afford any 

 criteria. 



In the anomalous cases of Acronycta referred to above, it seems 

 to me that we have proofs of the manner in which cocoons 

 have come to be dispensed with in certain Bombycids. That de- 

 fective nutrition has been a principal cause almost amounts to a 

 conviction. A parallel case is cited by Stretch in his " Zygaanidae 



