BREED: 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, tri.) 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. 
b. METAMORPHOSIS OF LARVAL MUSCLES INTO 
(1) Muscles of the Wing Type. 
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 
