THE AMERICAN ENTOMOLOGIST. 



63 



nate sutures of the soft joints of the chry- 

 salis, and his happy and circumspect ac- 

 count, from observations made on Vanessa 

 urticce, has formed the basis for subsequent 

 accounts, or, at least, no one obtained a 

 deeper insight into the philosophy of the 

 act until, some two years since. Dr. J. A. 

 Osborne, of Milford, England, discovered 

 that a distinct membrane is concerned in 

 it. In casual observations of the process 

 I had long become convinced that the 

 popular accounts were crude and inaccu- 

 rate, and had preserved specimens in the 

 act of transforming, for future study ; but 

 the philosophy of the change cannot be 

 satisfactorily made out from alcoholic 

 specimens alone, nor from the study of 

 one species. The present paper is based 

 on observations made on species in more 

 than a dozen genera, the conclusions hav- 

 ing been partially presented last June to 

 the Philosophical Society of Washing- 

 ton. 



The body of the larva is composed 

 (exclusive of the head) of 12 segments 

 or joints , and a subjoint. It is with 

 this subjoint that we have here to deal, 

 for to it are appended the anal prolegs 

 beneath, and the anal plate above the 

 rectum. 



If we carefully examine the anal plate 

 of the larvae of the true suspensi, we shall 

 find that while it differs in form it has 

 one feature in common, viz., the being fur- 

 nished dorsally and posteriorly with nume- 

 rous short spines and points, generally re- 

 trorse, or so placed that the larva can make 

 use of them in suspending. The special 

 spines on the anal plate are only fully de- 

 veloped after the last larval molt, being 

 more or less obsolete in the earlier stages, 

 and they are also under muscular control. 

 Even in the succincti, where, as a rule, the 

 anal plate is not specialized, spines are 

 nevertheless sparsely found, especially on 

 the hind border. 



All writers that I have consulted speak 

 of the larval suspension being due to the 

 entanglement of the hooks of the anal 

 prolegs in the silk, and do not mention 

 the use of the anal plate, for which the 



hillock of silk is sometimes spun in spe- 

 cial form.* 



The normal form of this hillock may be 

 likened to that of an inverted settee, or to a 

 shoe, or to a ship knee, and one of the 

 most interesting acts of the larva, prelim- 

 inary to suspension, is the bending and 

 working of the anal parts in order to fasten 

 the back of the plate to the inside of the 

 back of the settee, while the crotchets of 

 the legs are entangled in the more flattened 

 position or seat. In some cases (as in 

 Danais, Fig. 70, a) the hillock of silk is more 

 elongate, and the spines of the truncate 

 plate occur most around the lower margin, 

 and even beneath it, so that in fastening 

 them the larva seems to be drawing the 

 silk up the rectum. In other cases (as in 

 Etiptoieta) the plate, in addition to the 

 spines, has a prominent tubercle on each 

 anterior outer border, well calculated to 

 lock securely into the silk. After suspen- 

 sion, and as the fluids gravitate anteriorly, 

 the silken hillock becomes more conical 

 (the threads being loosely spun and elas- 

 tic), and the hooks both of the plate and 

 the prolegs hang more loosely from it. 



In the final getting rid of the larval 

 skin and attachment of the chrysalis there 

 are concerned : 



I St. Certain features belonging to the 

 larva and cast off with its skin ; 2d. Those 

 belonging to the chrysalis ; and to in- 

 telligibly explain the process it is neces- 

 sary to more fully characterize and ho- 

 mologize those parts than has hitherto 

 been done. 



In the former category, in addition to the 

 natural adhesiveness of the moist and mu- 

 cous separating membrane, there are three 

 physiological factors concerned : ist. The 

 tracheal ligaments (Figs. 66, 68, //) or the 

 shed tracheae from the last or ninth pair 

 of spiracles, which uniformly become blind 

 or obsolete in the chrysalis; 2d. The rectal 



* It is an interesting fact in this connection that Rosel, who 

 has never had any superior as a delineator of insect larvx, 

 makes the Nymphalids in his figures all suspend to an elon- 

 gate, conical piece of silk, apparently issuing from the anus, 

 with the legs invariably free and in no instance hooked. It is 

 evident, however, from his text, that he was not aware of the 

 use of the anal plate, and since he speaks of the larvae attaching 

 themselves by the hind legs or extremity, it is equally evident 

 that his figures do not correspond with the text ; while the 

 freedom of the legs in his figures is, of course, an error. 



