THORAX OF THE BLOW-FLY. 23 



rison of the two insects convinced me that it corresponds with the 

 upper or external portion of the vertical surface which Mr. Lowne 

 (op. cit.) calls the metathoracic tergum. For this reason also, 

 therefore, I must hold this to be a mistake, and that the external 

 portion at least of the surface in question belongs to the meso- 

 and not to the metathorax. 



As to the Muscular and Nervous Parts. — But fully as cogent 

 as either of the foregoing considerations is the evidence to be 

 derived from an examination of the muscular structure. The 

 nervous system of insects presents, to some extent, the repetition 

 of parts observable in the integument. There is generally in the 

 larva a pair of ganglia with corresponding nerves for each seg- 

 ment. Owing, however, to the concentration of the nervous 

 centres in the thorax of the imago (a concentration which, in the 

 Diptera, is carried to an extreme point), and their consequent 

 fusion into one large nervous mass, it is less adapted to the study 

 of homological relations than the muscular structure. The latter, 

 however, appears to me so obvious and so comparatively easy a 

 means of discrimination, that any diagnosis of external relations 

 that does not take it somewhat into account must of necessity be 

 pro tanto imperfect. The subcuticular muscles of larvae present 

 a very uniform repetition. Each segment has its own set of mus- 

 cles distinct from those preceding and following it. I will not 

 say that such a thing never occurs as the existence of a muscle 

 extending across two or more segments, for I know at least of 

 one instance in which this certainly appears to be the case*; 

 still, as a rule, observable not less in the imago than in the larva, 

 each segment is provided with its own muscles ; and the connate 

 condition or any approximation to it of two or more segments is 

 not, so far as I know, accompanied by any fusion, either real or 

 apparent, of their respective muscles. 



To illustrate this, it will be necessary to mention that the tho- 

 racic muscles of insects assume two different principal directions t, 

 a longitudinal and a lateral or vertical one. The former occupy 



* This occurs in the larva of the Crane-fly ; and a similar instance is men- 

 tioned in Sir John Lubbock's paper "On the Muscles of the Larva of Pygcera 

 bucephala " (Trans. Linn. Soc. vol. xxii. p. 174), being that marked No. 2 in the 

 first Plate attached thereto. 



t For a more complete account of the muscular structure of the thorax, see 

 " Essai sur le Vol des Tnsectes," par J. Chabrier, in Memoires du Museum d'His- 

 toire Naturelle, p. 410. 



