bkeed: metamorphosis of the muscles of a beetle. 349 



imagines of Coleoptera (compare Aubert, '53) : musculus metanoti, 

 musculus lateralis metanoti, musculus lateralis metathoracis, flexor coxae 

 metathoracis (secundus), extensor alae magnus metathoracis, and exten- 

 sor alae parvus metathoracis. 



a. Muscles that pass unaltered from the Larva to the Imago. 



The larval muscle fibres of Thymalus have the structure of this type 

 of cross-striated muscle. Cross and longitudinal sections are shown in 

 Figures 16, 22 (Plate 6) and Figure 33 (Plate 7). A granular sarco- 

 plasm containing the nuclei is found unevenly distributed just beneath a 

 well-marked sarcolemma. Occasionally the nuclei are embedded deep 

 in the fibres, but these exceptions are practically limited to a certain few 

 muscles ; as, for instance, the adductor mandibularis, where the fibres are 

 larger than usual and frequently have their nuclei embedded in the 

 contractile substance. The cross striations are well marked (Figure 33), 

 and may show all of the usual bands (Z, E, N, J, Q, H of Rollett, '85). 

 The muscle columns are flattened and of irregular shapes, so that the 

 Cohnheim's areas seen in cross sections (Figures 16, 22) make a peculiar 

 pattern. 



The trachae supplying the larval muscles break up into fine intracel- 

 lular tracheoles at the surface of the fibres. Whether these tracheoles 

 penetrate the sarcolemma or not, is difficult to determine with the 

 methods used. From cross sections (Figures 16, 22, trl.) it appears as if 

 they penetrated the sarcolemma (sar'lem), but remained in the super- 

 ficial layers of the sarcoplasm (sar'pl.). 



The muscle fibres of the abdomen, whose anatomical positions have 

 been described on page 338, preserve the structure just described in all 

 of the stages of the pupa and the imago. 



6. Metamorphosis of Larval Muscles into 



(1) Muscles of the Wing Tijpe. 



a. Period of the resting Larva or Period of Destructive Changes. In 

 the feeding larva the muscles which metamorphose into imaginal 

 muscles of the wing type show the same structure as the larval mus- 

 cles described above. When the larva ceases feeding, and the wings 

 have been evaginated from their hypodermal pockets, these muscles 

 undergo several rapid changes. Perhaps the most striking of these 

 changes take place in the contractile substance. This, in the course 

 of a few days, divides lengthwise into from four to ten strands, the 



