288 Memoir upon the compound 



one and the same plan ; and although it undergoes fewer modi- 

 fications than all the other systems, we see it exhibit peculiarities 

 wiiioh evidently announce the absence of a centre of action in 

 insects. 



While this mennoir was printing, I ascertained that the per- 

 fect insects which exist in water, exhibit some peculiarities in 

 their compound eye. Thus, as I have already remarked, this 

 eve is alwavs of a dark colour, which is owing jointly to the 

 thickness of the tunic of the cornea, to that of the varnish of the 

 choroid, and finally, to that membrane itself. The varnish of 

 the choroid, seen at its greatest thickness, appears to be of the 

 deepest black ; but when we stretch it, and reduce it to thin layers, 

 it passes to the darkest violet. The thickness of the choroid is 

 very remarkable in these species, particularly in the dydcee and 

 the hydroplujlce. In both, the circular trachea is totally wanting : 

 its place is occupied by a common trunk, from which issue two 

 principal branches, which proceed to form around the optic 

 nerve a network of tracheae larger than the branches which fur- 

 nish them. Finally, 1 have remarked that in most of the insects 

 which live in water, the retina is very near the cornea, a dis- 

 position which is also observed in the species which are not pro- 

 perly lucifugce, but which habitually fly at sunset only, such as 

 the s^eotnipcB. 



I have already observed that certain species presented several 

 different shades in their compound eye ; but, what is most 

 singular, there are some in which this organ exhibits in its mid- 

 dle part a very narrow stripe of a colour totally different from 

 the rest of the eye. This stripe, generally of a deep tint, is 

 almost always placed in the middle of the eye, so that it sepa- 

 rates it into two equal parts. But why is the tunic of the cornea 

 thus of a different colour ? It is difficult to account for this. 

 This disposition, hou'evcr, is not very rare ; we see it even very 

 decidedly in the locusla gigantea, the phasma rossia, as well as 

 in certain species of syrphiis and tabanm. The locusta lili- 

 folia * presents even several parallel stripes of a reddish brown, 

 whereas the rest of the eye is of a whitish green : what is most 

 particular is, that its stripes do not exist in the middle of the 

 eye. I have described in the course of this memoir the com- 

 pound eyes of the cloportce, ihinkms; that they were true insects, 

 i. e. articulated animals deprived of a circulation of blood, but 

 furnished with tcacheee. Subsequently I have made new re- 

 searches as to the organs of circulation of a great number of 



* All that has been hitherto said on the locusta lUifulia ought to be re- 

 ferred to the locusta faicala. These two names liave been confounded by 

 mistake. 



jnvertebral 



