210 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Kingslcy appear to make it certain that tho ommatoum is a single layor 

 of cells, and consequently Reichcnbaeh's crystalline-cono layer represents 

 tlio whole ominateuin, and corresponds in its oarly stages to tho optic 

 thickening of Vespa. If this bo so, it is clear that tho outer wall of 

 Reichcnbaeh's " Augenfaltc " cannot dovclopo into the layer of retinula) 

 and rhabdoms. It is probable that tho " optic invagination " of Kingsley, 

 the " Augcnfalto " of Reichenbach, and tho ganglionic fold of Patten arc 

 one and the same thing. 



To show that his view as to tho threo-laycred nature of the ancestral 

 Arthropod eye is correct, it was important for the author to demonstrate 

 that the eyes of Dytiscus and related forms, which have been described 

 by Grenadier as open cups, and as the simplest type of Arthropod eye, 

 aro really closed vesicles primarily composed of three layers of cells. 

 Tho author has examined tho ocelli in tho larvae of Hydrophilus, Dytiscus, 

 and Acilius ; in tho last of these there are six pairs of cells, of which 

 the two dorsal pairs are very deep, and resemble the two-layered ocelli 

 of certain spiders ; the space between tho lens and retina is completely 

 filled by a layer of very long colls — corneagen — whose deep nucleated 

 ends are somewhat swollen and bent away from the centre of the eye ; 

 they are so arranged that there are no nuclei of the corneagen just 

 above the centre of the retina, while there is a distinct layer of them 

 over its periphery, as well as on the walls of the inner half of the eye. 

 The periphery of tho corneagen contains a thin layer of very large dark 

 globules, many of which contain a still darker corpuscle ; this layer of 

 pigment-like bodies extends from the edge of the lens to the retina. 

 The floor of the eye is formed by a layer of upright retinal cells, each 

 provided with a double rod, and there is a median furrow. 



In Hydrophilus, the ocelli are formed by invaginations of the 

 ectoderm directed diagonally inwards, and tho ocellus is composed of 

 three distinct layers of cells, of which the thick inner layer, the retina, 

 is directly continuous on the dorsal side with tho hypodermis. The eye 

 is not really but only practically a closed vesicle, as is shown by the 

 absence of nuclei at ono point and the continuity of the three layers. 

 Dr. Patten comes to tho conclusion that there are ocelli in the larvae of 

 insects very similar to what, in a former paper, ho regarded as the 

 ancestral eye of Arthropods. 



Taking a more extended survoy, tho author finds himself led to the 

 supposition that tho dorsal and ventral eyes of Phronima and Gyrinnus, 

 and those of the males of Bihio and Cloe, as well as the dorsal and 

 ventral parts of the eyes in Libellulidee and Euphausia, are homologous 

 with the dorsal and ventral halves of the larval compound eyes of Vespa. 

 Tho parts of the compound eyes of Vespa, and in all probability of most 

 other insects, are in turn homologous with the posterior upper ocelli of 

 Acilius and their dorsal extensions. In such cases as those seen in the 

 larva? of Corethra and Phryganids, the ocellus has already become a 

 compound eye, whilst its dorsal extension does not attain that perfection 

 until the imaginal stage is reached. 



a. Insecta. 



Dermal Sensory Organs of Insects.*— Herr O. v. Rath has a pre- 

 liminary notice of his investigations into the structure of the dermal 



* Zool. Anzcig., x. (1887) pp. 627-31, 645-9. 



