THE DEVELOPMENT OF THE COMPOUND EYE. 541 



characteristic of the Ametabola. In ma-ny insects, as the 

 noctuid and ; crepuscularian Moths (Lcpidoptem heterocera), the 

 retina is segregate that is, groups of ommatea have distinct 

 nerves, just as in the larval Neuroptera (Libelhila). In the 

 Muscidse and the imago of the Libellulidae the retina is always 

 continuous, and a single thick decussating nerve supplies the 

 whole eye. In the Gnats and Tipulidas (Diptera nematocera) 

 the retinae are always, so far as I know, segregate, and the 

 compound eyes come into functional activity in the active 

 nymphs in the aquatic species. The several stages in the 

 development of the continuous retina of the Dragon-flies 

 (Libelhila) are represented in Fig. 70. 



Historical and Critical. Recent writers on the development 

 of the compound eye in Arthropods have almost entirely 

 devoted their attention to an attempt to form a plan or 

 scheme which shall serve as a morphological type to bring 

 into accord the various kinds of compound eye, which they 

 believe exist in Arthropods. 



The statements made are chiefly supported by little evidence 

 and much argument ; some, as Carriere [232] and Parker [250], 

 maintain that the whole structure originates in a simple 

 thickening of the epidermal layers, and there is no doubt, as 

 far as the dioptron is concerned, this statement is partly 

 correct. Others, as Bobretzki [253], Reichenbach [255], 

 Kingsley [256], and Patten [239], state that it is formed as an 

 invagination of the epidermic layer. That the eye is situated 

 in many Arthropods, both Crustacea and Insecta, either per- 

 manently or temporarily in an invagination of the integument 

 is indubitable compare for example the eye in Apus (Bernard 

 [257]), and the invaginated eye-disc of the dipterous larva so 

 that, in some Arthropods at least, the truth is apparently on 

 the side of the invagination theory. All these observers, how- 

 ever, think that the eye is developed from the whole invagina- 

 tion, which becomes, according to them, flattened out, so that 

 three layers are subsequently formed, and they are all at 

 variance as to what becomes of these layers. According to 

 Reichenbach, the cavity of the vesicle disappears at an early 



