OF THE LARVA OF PYGÆRA BUCEPHALA. 185 
No naturalist had previously undertaken this task, though Lyonet has mentioned here 
. and there certain variations observed by him. 
It would have taken a great deal of time, and seemed to me scarcely worth while, to 
make this comparison in all the segments of the body ; and I have therefore confined 
myself to the third, fourth, fifth and sixth, thus including a thoracie somite, two abdo- 
minal without prolegs, and one with these appendages. 
These four segments I compared carefully in nine different specimens, five of which 
were fed upon Oak and four upon Beech. But in order to determine a few minor 
points, I dissected perhaps as many more specimens in a cursory manner, which how- 
ever confirm me in the opinion that the larger muscles scarcely vary at all. Indeed 
the only mutation observed in them at all worthy of notice was that in one case the 
muscles of the third abdominal segment, 16, 17 and 18, were not attached to the skin 
in front, but were continuous with the corresponding muscles of the preceding segment. 
The differences which do exist are not exactly those which I expected to find; whilst 
some things which I rather thought might vary, remain on the contrary always the 
same. 
Let us take for example Nos. 4 and 5. In the first abdominal segment 4 passes on the 
inner side of 5; the contrary, however, might apparently have been the case, without 
alteration of the places of insertion, or of the mode of action of the two muscles; and I 
thought it probable, therefore, that 5 might sometimes lie on the inner side of 4. 
But although six segments in at least fifteen specimens were examined, making in all 
ninety instances, the arrangement was invariably the same in every instance. 
This remark is applicable to several other muscles, for instance 6 and 8, 7 and 8, and 
23 and 18 in the third abdominal segment; and, indeed, I never observed a case in which 
a muscle varied in this respect in its relation to another. 
The muscles 4 and 5, however, offer a different arrangement in the thorax to that 
which is so constant in the abdomen. Here, if they are rightly identified, which perhaps 
` may be doubted, though I believe that I am correct, 5 is completely double and lies on 
the inner side of 4. 
This altered arrangement appears to be necessitated by the altered position of the 
fore end of 4, and by the presence of 60 and 61. : 
Again, 8, which in the following segments lies on the inner side of 11, in the first 
thoracic segment has shifted its position and passes on the outer side. 
A careful examination of the Plates will show a few more instances of this fact, which, 
however, are not very frequent. : diei) 
The relative sizes of the different muscles appeared to vary very little in different 
specimens, except indeed in those muscles which might be termed the variable 
muscles. i : 
Such were especially 12, 13, 14 and 15; 28, 29 and ws 56; 59 and 60; anis m 
lesser degree, 31, 32, 33 and 34; 57 and 58; 11; 23 and 24 in the third thoracic segment, 
8 in the third thoracie segment, and 40, 41 and 42. 
If we suppose, as seems probable, that muscles composing 
usually act together, and that the same is the case with the oth 
the series 12, 13, 14 and 15 
er three series, 28, 29, 50 ; 
