ZOOLOGY. 327 



form a segment of a framed arch or truss, which cooperates with the 

 external shell in sustaining the weight of the body ; the necks of the 

 t\vo femora forming together opposite segments of an arch. The first 

 and second of these conditions have been frequently adverted to by an- 

 atomical writers, but the third has almost invariably escaped obser- 

 vation. 



As regards the structure of other bones, Dr. Wyman also shows in 

 what direction force or weight is applied to them, and in what direc- 

 tion the cancelli are arranged within them. On the lumbar vertebra 

 there is vertical pressure; within, the principal fibres are also vertical. 

 On the neck of the thigh-bone the weight of the body is applied ob- 

 liquely to the end of an arm ; within it, there is a combination of fibres, 

 giving strength with lightness, which forms a frame mechanically 

 adapted for resisting the w r eight which rests upon it. On the astraga- 

 lus the pressure again is vertical, but this bone rests on two others, 

 one below it, the os calcis, and the other in front, the scaphoides ; 

 within there exist two series of cancelli, directing the pressure on 

 the surfaces of support, and very nearly the same description applies 

 to the os calcis. A certain direction of fibres in all these instances 

 coexists with a certain direction, or certain directions, of the transmis- 

 sion of pressure. From this constant association of structure and 

 function, the inference seems unavoidable, that they are means and 

 ends. Proc. Bost. Soc. Nat. Hist. 



METAMORPHOSES OF THE LEPIDOPTERA. 



PROF. AGASSIZ said that he had during the past season been 

 studying the metamorphoses of the Lepidoptera, and, to his great 

 surprise, he had found that one stage in the transformation of these 

 insects has been overlooked by naturalists. We knew the Lepi- 

 doptera in three conditions, that of the worm, furnished with jaws 

 and jointed, the chrysalis, and the perfect insect, with four wings. 

 The change not before described, which he had noticed, is some- 

 what concealed under the skin of the caterpillar. The animal at a 

 certain period swells at the thoracic region, and becomes extremely 

 sensitive to the touch in this part, the skin being, in fact, in a state of 

 inflammation. On cutting open the skin at this place, Prof. Agas- 

 siz found beneath it a four-winged insect, before it had passed into 

 the chrysalis state. The wings were long enough to extend half 

 the length of the perfect insect, The posterior pair he found to be 

 membranous bags, somewhat flattened, like the respiratory vesicles of 

 marine worms, with distinct ribs, which are blood-vessels. The an- 

 terior pair are also bags, with their upper half stiff and inflexible, 

 like the elytra of Coleoptera. The legs are tubular, but not joined, as 

 in the perfect insect, The jaws are changed into two long tubes, 

 which are bent backwards, as are also the antennae. In the chrysalis 

 the wings are flattened and soldered together, as are the legs and 

 sucking-tubes, which are bent backwards. The order of development 

 of the different parts, and the coleopterous condition at an incomplete 

 stage, show that naturalists have been in error in placing chewing in- 



