breed: metamokphosis of the muscles of a beetle. 321 



June. As long as they were under observation, i.e., till the first part of 

 July, they showed no signs of leaving the protected places about the 

 fungus from which they hatched. Inasmuch as the Polyporus which 

 serves the larvae as a food plant is an annual, there is probably but one 

 brood during the year, the eggs not being deposited until fall. 



Thymalus is a particularly good form for histological study, inasmuch 

 as material seems to be plentiful wherever there is a food supply. It is 

 of convenient size and has a relatively thin cuticula at every stage. 



2. Methods. 



Since Thymalus is a small beetle, it has been necessary in studying 

 the anatomy of the musculature to resort to reconstructions from sections 

 in place of dissections. Material killed in hot water, or by some method 

 which gave no distortion, was used, and serial sections cut 16| ^u. in 

 thickness. To obtain a plane for reconstructiou, a " definition appa- 

 ratus " made by Zimmermann has been used. By means of this apparatus, 

 the lateral faces of the paraffin block were cut exactly perpendicular to 

 each other and to the proposed plane of sectioning. Two adjacent lateral 

 surfaces were then painted with a mixture of soft paraffin and lampblack, 

 melting at about 51° C, after which each face was again trimmed in the 

 " definition apparatus " so that only a very thin layer of paint was left. 



The sections were cut on a Minot microtome in a plane perpendicular 

 to that of the painted surfaces. In mounting the sections, much of the 

 lampblack washes away, but, with ordinary care in the staining and 

 other processes, enough adheres to the albumen affixative to give a very 

 definite line at the outer edge of the lampblack area. A magnification 

 of 120 diameters was used in all of the reconstructions, as this made the 

 thickness of each section equivalent to 2 mm. The drawings made from 

 the reconstructions have been reduced to -^g of their original size in the 

 process of reproduction, so that the ultimate magnification in the plates 

 is about 67.5 diameters. 



Whole and partial preparations have been used in checking the results 

 of reconstruction. 



3. Anatomical Changes of the Muscles. 



Early in my study of the histological alterations of the muscles in 

 Coleoptera, it was found that all of the muscles do not undergo the same 

 changes. Some remain unchanged from larva to imago, many metamor- 

 phose, and a few degenerate. Whether or not there were any newly 



