INSECTA. 



961 



Wagner, who has found that the fibre is pro- 

 longed as a shealh over the sides of the cone, of 

 which it is supposed to form a part. In this 

 manner, each of the thousands of coraeules or 

 facets that form the compound eye, transmits 

 impressions from without inwards to the optic 

 nerve and brain, the perception of each being 

 confined to that of the object immediately 

 before it, or in a line with its axis of vision. 

 On the exterior surface, between each cornea, 

 there are often some very fine hairs, as on die 

 cornea of the bee, which Burrneister likens to 

 eyelashes, and thinks that they assist to con- 

 fine the field of vision, as well as protect the 

 cornea. This is the usual structure of the eye. 

 In sMelolontha, Straus-Durckheim describes 

 the filament of the optic nerve as passing 

 through a second or comttion choroid, and as 

 afterwards uniting to form a general retina, 

 which is connected with the optic nerve by 

 means of short thick columns. The use he 

 assigns to this structure is that of intercepting 

 the impressions of light, which might otherwise 

 be too powerful. 



The ocelli, or simple eyes of insects, re- 

 semble those of Arachmdans,* in being 

 formed of a very convex, smooth, single cornea, 

 beneath which is a spherical crystalline lens, 

 resting upon the plano-convex surface of 

 the expanded vitreous humour, the analogue 

 of the transparent cones of the compound eyes. 

 The vitreous humour, as in Arachnidans, is 

 more convex on its posterior or under surface, 

 and is contained in the expanded retina at the 

 termination of the optic tubercle, upon which 

 each ocellus is situated, the exterior surface of 

 the retina being covered by a dark pigmentous 

 membrane, the proper choroid, which is re- 

 flected inwards upon the anterior portion of the 

 vitreous humour, to form the iris and pupillary 

 aperture. Dialler, who discovered this ^struc- 

 ture in the stemmata of insects as well as 

 Arachnidans, concludes that the function of 

 the simple eyes is confined exclusively to the 

 perception of near objects, and that of the 

 compound eyes to more distant ones, and has 

 given many facts in illustration of this opinion, 

 and which shew that in many instances, parti- 

 cularly in the Orthoptera, the ocelli are so 

 placed as to render it almost impossible that 

 they can be used except in viewing near ob- 

 jects. In all insects that undergo a true me- 

 tamorphosis ocelli constitute the only organs 

 of vision in the larva state. They vary in 

 number in different species; thus in the active 

 larvae of Hymenoptera, as in Athalia, there are 

 only two, one on each side of the head ; this is 

 also the number in some of the carnivorous 

 Coleoptera. But in others there are six on 

 each side, as in Dyticus, and the same number 

 is found in most of the Lepidoptera. We have 

 recently detected what we believe to be organs 

 of vision in a Dipterous larva, CEstrus avis, (Jig. 

 360,) which resides in the frontal sinuses of the 

 sheep, into which, probably, a small amount of 

 light may enter through the nostrils. These 

 consist of two brown spots on each side of the 



* See ARACHNIDA, vol. i fig. 94. 



VOL. II. 



head, (h 2,) placed al a little distance from each 

 other, immediately beneath a convex and very 

 transparent part of the tegument, which resem- 

 bles a true cornea. This is the most simple 

 form of eye we have yet met with in insects, 

 and seems to be merely for the perception of 

 light, like die eyes of the Medusa: discovered 

 by Ehrenberg, but perhaps more organized, as 

 the spots observed appear to be a choroid, 

 which is seen to descend until it is lost in the 

 substance of the part. No compound eyes 

 exist in any larva that undergoes a complete 

 metamorphosis. In those which undergo an 

 incomplete one, as in the Orthoptera, the 

 facets of the eye are larger and more convex 

 than in the perfect state, and the true ocelli 

 which exist in the perfect state are not deve- 

 loped. In the larva and pupa of Reduvius 

 personatus there is an aggregation of simple 

 eyes, like those of Myriapoda, very much 

 larger and more convex than the facets of the 

 compound eye of the perfect insect. Simple 

 eyes exist in the perfect state in the Hymenop- 

 tera, Orthoptera, Hemiptera, Neuroptera, Tri- 

 choptera, Homoptera, and in some of the Lepi- 

 doptera and Homaloptera, and in a very few 

 instances in the Coleoptera. 



Organ of hearing. Every naturalist who has 

 at all attended to the consideration of the faculty 

 of hearing in insects, is doubtless convinced 

 that these little creatures are not merely affected 

 by sounds, but that hearing constitutes one of 

 their chief senses; yet it is hitherto undecided 

 what organ or part of the animal is the seat of 

 this function. We have above stated our opinion, 

 (p. 892,) with many others, that it resides in the 

 antennae, which, if this be not confirmed, is at 

 any rate supported by the experiments hitherto 

 made upon these organs, and also by their 

 structure and the manner in which they are 

 employed by the insect. The nerves distributed 

 to the antennae have often a ganglion at their 

 base, and are divided into many branches almost 

 immediately after they have entered the organ, 

 so that at present no difference has been detected 

 between the distribution of the nerves to these 

 parts and those to other structures. They cer- 

 tainly exhibit no bulbed extremity like the audi- 

 tory nerves of the higher animals, while the 

 manner in which the antennae are employed 

 by many insects has induced some observers to 

 believe that they are simply organs of touch. 

 This cannot be their primary function, since, as 

 formerly remarked, they are too short to be 

 employed as tactile organs by many insects, 

 while their structure, we conceive, is in every 

 instance adapted for hearing or perceiving the 

 pulsations of the atmosphere. 



Organs of touch. The organs which appear 

 specially adapted to the exercise of this function 

 are the palpi, which derive their nerves, as 

 above shown, from the medulla oblongata. 

 These organs are employed in a similar manner 

 by all insects to touch the food. It is with 

 these that the insect, as it \\ere, Jeels about 

 when it is in search of nourishment, and hence 

 these may be regarded as the proper tactile 

 organs. It has sometimes been supposed that 

 they are also concerned in the function of taste, 



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