vi ANTENN AT A SENSORY ORGAXX 469 



the upper lip. From the ganglion frontale an unpaired median nerve, the nervus 

 recurrens ///, runs back under the brain along the dorsal wall of the n.sophagus, 

 and enters an unpaired stomach ganglion in front of the masticatory stomach. A 

 lateral pair of nerves arises from this, 2 small ganglia occurring in their course. 



The paired' visceral nervous system consists of two pairs of ganglia gn and <//>, 

 which lie on the oesophagus, the anterior pair being covered by the brain. These 

 nerves are connected with each other, with the nervus recurrens, and with the brain 

 by anastomoses. The nervus recurrens and the paired ganglia give off nerve 

 branches to the oesophagus and to the salivary glands. 



Besides the above nerves insects may have sympathetic thoracic and abdominal 

 ganglia either paired or unpaired. The paired portion of the visceral system may 

 also be wanting. 



In the Lepidoptera, close above the abdominal portion of the ventral chord, is 

 found a longitudinal strand of connective tissue, which seems to be a formation of 

 the Neurilemma of the ventral chord. Muscles are attached to this which run to 

 tin- neighbouring ventral exoskeleton. This strand, whose significance is not yet 

 sufficiently explained, has been called the chorda supra spinale. It has nothing to 

 do with the vertebrate chorda. 



VI. Sensory Organs. 



A. Eyes. 



We can distinguish single -lensed eyes or ocelli from compound 

 eyes (facet eyes). The Myriapoda have ocelli generally in large 

 numbers grouped closely together on each side dorsally. Only 

 Scutiyem has a compound eye on each side : this eye, however, differs 

 in structure in many respects from the compound eye of the I used a. 



Most adult Hexapodu have ocelli as well as facet eyes. The small 

 ocelli then generally lie in threes on the frontal region between the 

 two large facet eyes. The larvse have ocelli only, these often occurring 

 in great numbers. Ocelli are seldom found alone (i.e. without facet 

 eyes) in adult Hej-ujxid", but this is the case in the Collembola among 

 the Apterygota, and in lice (Pediculidce) and fleas (Aplianiptem}. Ocelli 

 are wanting in adult Dermpter<i, and among the Orthoptera in the 

 Locustidce, among the Itlujnclmtn in the Hydrorora, among the Lepiilopfn-n 

 in the Gebmetrina and Ifhu^nlnr, /-n, which thus as adults possess only 

 facet eyes. 



Structure of the Ocelli. The simply constructed ocellus of the 

 Diltiscu* larva is very instructive (Fig. 328). Above the ocellus the 

 chitin thickens into a lens. Below it the hypodermis is depressed in 

 the form of a pit. The hypodermis cells standing at the base of this 

 depression form the retina of the eye. Each retinal cell is connected 

 with a nerve fibre, contains pigment, and produced outward towards 

 the lens in the form of a rod. The cells at the edge of the depression 

 are free from pigment at their outer ends, and push their way in 

 between the retina and the lens, filling it up and forming a sort of 

 vitreous body. The ocelli of other Inseda and Myriapida are similarly 



