ANA TOMY AND MORPHOLOG \ ' OF INSECTS. 15 



greater number is characteristic of the Orthoptera. In the 

 -Diptera there are nine abdominal somites in the embryo, 

 although the number of obvious segments in the perfect insect 

 is usually greatly reduced. 



A pair of nerve ganglia are developed from the cells which 

 form the inner surface of each post-oral somite. These ganglia 

 are united by a pair of nerve cords, longitudinal commissures, 

 and also by transverse commissures between the ganglia of 

 each pair. 



In all the higher Insecta, and more especially in the true 

 flies, there is subsequently a great concentration of these 

 ganglia, which lose their relations with the somites in which 

 they are first formed and become united into a single complex 

 centre. 



The longitudinal commissures are continued round the 

 stomodaeum and terminate in the cephalic nerve-centre, the 

 brain, which is developed from the cells forming the inner 

 surface of the procephalic lobes, and consists therefore of two 

 symmetrical halves. Hitherto great discrepancies have existed 

 in the \vorks of comparative anatomists with regard to the 

 number of somites which take part in the formation of the 

 head. This arises from the attempt to regard the whole head 

 as segmented. Thus some speak of five, and others of six, or 

 even seven, cephalic segments, making two, three, or four of 

 these pre-oral, and homologising the great eyes, the antennae, 

 and even the upper lip and the ocelli, with the thoracic and 

 stomal limbs (mandibles and maxillae). 



Development lends no probability to such views, as there is 

 no segmentation of the pre-oral region. It is true the antennae 

 present some similarity to the ventral appendages, and the 

 stalked eyes of the higher Crustacea give an appearance of 

 truth to the hypothesis that the eyes and antennae are modified 

 limbs. As both are developed from the procephalic lobes, and 

 the latter at least do not at any period present the faintest 

 resemblance in position or mode of origin to the ventral appen- 

 dages, I am unable to see any reason for regarding either as 

 homologous with the post-oral appendages. 



