HARDWICKE'S SCIENCE-GOSSIP. 



247 



This central framework is invested by cortical 

 ganglionic cells, which possess distinct nuclei and 

 nucleoli. A special cellular mass forms a cap to each 

 pair of calices, and this consists of smaller cells 

 without nucleoli. Above the meeting-place of the 

 trabecule is a peculiar laminated mass, the central 

 body, which consists of a network of fibres continuous 

 with the neighbouring ganglionic cells, and enclosing 

 a' granular substance. The trabecule, peduncles 

 and cauliculi appear to be a peculiar modification 

 of that constituent of the ganglia, which Leydig has 

 termed "dotted substance" {ftinkt-siibstanz). It 

 can be resolved by the highest powers of the 

 microscope into densely interwoven fibrillse, which 

 Krieger* has traced into both ganglion-cells and 

 peripheral nerves. In the antennary lobes the 



endings, and by peculiar sensory rods or filaments 

 upon the antennce. These are taken to be the 

 organs respectively of sight, hearing, and smell. 

 Insect eyes vary conspicuously in structure ; the 

 auditory organ is found in places so dissimilar as 

 the tibia of the fore-leg and the first abdominal 

 segment. Other sense-organs, not as yet fully 

 elucidated, may co-exist with these. The maxillary 

 palps of the cockroach, for example, are continually 

 used in exploring movements, and may assist the 

 animal to select its food ; the cerci, where these are 

 well-developed, and the halteres of Diptera, have 

 been also regarded as sense-organs of some unde- 

 termined kind, but this is at present wholly con- 

 jecture.* 



Our scanty remaining space forbids us to describe 



^ '—/- t-fzrp=!^^ 



ft^i -4 



/•om 



-op 



a 



77/ 



com 



Fig. 148. — View from the outer side of the left half of model of upper part of brain of Cockroach. The oblique lines in this and 

 Fig. 149 indicate the successive slices of which the model is composed, their direction being that of the sections in Fig. 150. 

 op, cut end of optic nerve ; an, cut end of antennary nerve. 



Fig. 149. — Right half of model-brain seen from the inner side, with the parts dissected away, so as to show the anterior nervous mass, 

 a ; the median mass, m ; the mushroom-bodies, mb ; and their stems, si. The cellular cap, c, has been raised, so as to display 

 the parts below : com, is a part of the connective uniting the brain and infra-cesoph.Tgeal ganglia. [Figs. 148-150 are taken from 

 Mr. E. T. Newton's paper in " Journ. Quekett Club," 1879.] 



pitnkt-siibstauz appears in a more ordinary form as 

 a network of fine fibres enclosing ganglion-cells, 

 and surrounded by a layer of the same. It is remark- 

 able that no fibrous communications can be made out 

 between the calices and the cauliculi, or between 

 the trabecules and the cesophageal connectives. 



Each optic ganglion contains two lenticular bodies, 

 as in various insects and Crustacea ; the nerve-fibres 

 cross before entering the first of these, again between 

 the two, and a third time between the outer lenticular 

 body and the eye. 



The sense-organs of insects are very variable, both 

 in position and structure. Three special senses are 

 indicated by transparent and refractive parts of the 

 cuticle, by tense membranes with modified nerve- 



* In the crayfish, " Zeits. f. wiss. Zool.," Bd. xxxiii., p. 541 

 (1880). 



what has been made out respecting these various 

 organs. We can only notice one, and that briefly. 

 The most interesting and the best understood is 

 certainly the organ of sight. 



The compound eyes of the cockroach occupy a 

 large, irregularly oval space (see fig. 151) on each side 

 of the head. The total number of facets may be 

 estimated at about 1800. The number is very variable 

 in insects, and may either greatly exceed that found in 

 the cockroach, or be reduced to a very small one 

 indeed. According to Burmeister, the Coleopterous 

 genus ^Nlordella possesses more than 25,000 facets. 

 Where the facets are very numerous, the compound 

 eyes may occupy nearly the whole surface of the 

 head, as in the house-fly, dragon-fly, or gad-fly. 



* It is to be remarked that unusually large nerves supply the 

 cerci of the cockrciach. 



