503 THE ANIMAL KINGDOM. 



eye. The ocellus is found in the larvae of orders with complete metamor- 

 phosis (Metabold] and the larval Ephemeridae, and in addition to facetted 

 eyes in the imago of some orders, e.g. Hymenoptera, though wanting in 

 others, e. g. Dermaptera. It is the only kind of eye present in Collembola, 

 the Pediculidae, and Siphonaptera. The compound eye is present in the 

 imago of all groups with the exceptions just named and in the larvae 

 of Ametabola and Hemimetabola except Ephemeridae, though it may differ 

 in detail from that of the imago. The number of ocelli present is not 

 constant, but in the imago there are usually three. Of the polymeniscous 

 eyes there are two, one on each side of the head. They vary much in size, 

 are sub-divided in some Lamellicorn Coleoptera into two, e. g. in Geotrupes. 

 So too in the male Chloeon (Ephemeridae), in which they are slightly stalked, 

 especially the median halves. The ommateum of the ocellus is variably con- 

 formed. In the larval Dytiscus and Acilins (Coleoptera} it is monostichous 

 and apostatic. The visual (retinal) cells possess visual rods at their inner 

 ends, and the surrounding cells are pigmented at their peripheral ends, 

 hyaline at their inner ends, which bend inwards horizontally over the visual 

 cells. The structure requires examination in other larvae. In the ocellus of 

 the imago the ommateum is diplostichous, the vitreous cells are generally 

 small and flat, and the rods of the visual cells composed of two refractile 

 plates covering the anterior (or inner) fourth of the cell. The shape and 

 size of the lenses in the polymeniscous eye vary even sometimes in one 

 and the same eye, e. g. in Odonata. The vitreous cells either persist in 

 a uniform layer (the acone eyes of Coleoptera minus Pentamera, Rhynchota, 

 Tipularidae among Diptera, Dermaptera), or are grouped into fours forming 

 vitrellae, and then (i) surround a clear fluid or gelatinous cone (the/tfjfcfc* 

 cone eyes of Diptera Brachytira, e. g. Mused}, or (2) are reduced to feeble 

 remnants inclosing a solid crystalline cone composed of 2-5 parts, the 

 eucone eyes of other Insecta, including Lepisma. The visual cells are 

 grouped to the number of seven into retinulae. In acone eyes one cell 

 stands in the centre with the six others around, and the visual rods or 

 rhabdomeres are contained one in each cell. In pseudocone and eucone 

 eyes the seven cells are grouped round a central axis. The rhabdomeres 

 in the former coalesce only anteriorly, in the latter throughout their whole 

 length, into a rhabdome. Pigment cells lie between the vitrellae and 

 retinulae separating the ommatidia from each other. So far as observations 

 go the compound eye is autochromic. There are as a rule two optic 

 ganglia, a peripheral and a central. Details vary much 1 . Auditory organs 



1 Patten draws attention to the following points (Mitth. Zool. Stat. Naples, vi. 1886): (i) As 

 to the ocellus of the imago. The vitreous cells are, properly speaking, corneal hypodermis cells ; 

 the retinophorae are terminal ; and the layer of visual cells is a retineum in which the retinidia of 

 the retinophorae form a continuous layer. (2) As to the compound eye. It has the typical structure 

 detailed in note, p. 452. In Mantis religiosa, the type which he examined, the retinidia = crystalline 



