348 bulletin: museum of comparative zoology. 



The serial sections were cut 6| or 10/a in thickness and stained on 

 the slide. Borax carmine, safranin, haemalura, and several haematoxylin 

 stains, including iron haematoxylin, were tried, but none gave as good 

 results as a saturated aqueous solution of thionin. This is very selective 

 and does not stain the cytoplasm of the growing tissues as deeply as 

 most of the other stains. My thionin preparations have not faded 

 much, though some of them are three years old. The preparations in 

 which the stain has a greenish tinge fade more quickly than those in 

 which it is of a deep blue. All of the preparations used in making 

 drawings were stained in thionin. Haemalum and safranin are also 

 very satisfactory stains. 



2. Histological Clianges of the Muscles. 



The hypodermal muscles of insects exhibit three varieties which, 

 though fundamentally alike, present quite dififerent appearances under 

 ordinary magnifications. Weismann ('62) has designated these types 

 as the larval, the leg, and the wing muscles, from their principal 

 distributions. 



The muscles of the larval type include in Coleoptera not only all of 

 the muscles of the larva, but also some of those of the pupae and 

 imagines. Those found in the pupa and imago exist in the abdominal 

 region only, and are muscles of the larva which have persisted unaltered 

 during the metamorphosis. All of these muscles are composed of a few 

 relatively large fibi'es with a well-marked sarcolemma, and usually with 

 the nuclei at the periphery of the fibres. 



The muscles of the second, or leg, type are formed during pupal life, 

 and are found not only in the legs but also in other parts of the body. 

 In the imaginal form of Thymalus all of the skeletal muscles are of this 

 type, except the few metathoracic muscles mentioned below, and the 

 persistent larval muscles of the abdominal region noted above. These 

 muscles are composed of numerous small fibres frequently arranged in a 

 penniform or bipenniform manner and attached by a common tendon. 

 The nuclei are found at the surface of the fibres in Thymalus, but in 

 many other insects, including many Coleopterous forms, they are 

 arranged in rows along the axis of the fibres. 



The muscles of the third, or wing, type are frequently spoken of as 

 the fibrillar muscles, since they separate very readily into their primi- 

 tive fibrillae. They are composed of very large fibres with nuclei scat- 

 tered throughout their substance. Numerous tracheoles penetrate the 

 fibres of these muscles. The following muscles are of this type in the 



