364 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



formation in the pupa of other beetles; that is, from cells resembling the 

 tracheal cells, but probably having a diflerent origin. 



3. Observations on other Coleoptera. 



Bruchus obtectus Say, the common bean weevil, was chosen for com- 

 parison with Thymalus chiefly because of the different conditions which 

 might be expected in tlie leg muscles. Thymalus is a form with an un- 

 modified larva possessing six well-developed legs. Bruchus, on the other 

 hand, has a more highly specialized larva, which has legs when it hatches 

 from the egg, but at the first moult loses all except the merest rudi- 

 ments of them. During the remainder of larval life, these rudiments 

 are barely visible. The legs of the first larval form are scarcely larger 

 than the hairs which are found on other parts of the body. They do 

 not show all the joints of the adult leg, but only the femur and tibia, 

 the latter possessing an enlargement at the distal end which represents 

 the tarsus. In whole preparations, no muscles can be distinguished in 

 these legs, and it is probable that they are functionless as locomotor 

 organs. (For descriptions and figures of the larval stages of this insect, 

 see Chittenden, '99.) 



Sections of half-grown larvae — the youngest used in sectioning — show 

 rudiments of legs, at the bases of which are found masses of cells. Tliese 

 masses are principally composed of the small spindle-shaped cells which 

 later give rise to the muscles of the imaginal legs. These cells have a 

 somewhat oval nucleus surrounded by a small amount of cytoplasm. A 

 few tracheae aerate this mass, while an occasional leucocyte is also found. 

 The origin of the spindle cells has not been traced, but they are pre- 

 sumably the embryonic mesoderm cells which would have formed the 

 muscles of the legs, had muscles been functionally developed in the legs 

 of the larva. 



At the time of pupation, three kinds of cells are found in these masses. 

 There are (1) the leucocytes, which are readily distinguished. They are 

 several times larger than the other cells, have a more rounded form, 

 an abundant cytoplasm, and a spherical nucleus, in which the chro- 

 matin network lies chiefly at the periphery. The remaining cells are 

 spindle-shaped and apparently all alike ; but later stages of development 

 indicate that they are of two kinds, which probably have different origins. 

 These are (2) the mesoderm cells mentioned above and (3) mesenchy- 

 matous tracheal cells. The mesoderm cells probably have an embryonic 

 origin, and they develop into muscles. No direct proof of the origin of 

 the tracheal cells can be given, because in their young stages it has been 



