DISSECTED MODEL OF AN INSECT'S BRAIN. 153 



nervous matter, which passes forwards, to end abruptly upon the 

 front surface of the brain. The structure was correctly described by 

 Dujardin, in 1850, but does not seem to have been recognised by 

 Leydig ; this was, no doubt, due to the method of investigation em- 

 ployed by the latter, which consisted in clarifying the brain (pre- 

 viously hardened in alcohol) in potash solution, or glycerine. 



With regard to the origin of the nerves of the ocelli (o), it is 

 desirable that Leydig's figure should be verified, for it seems very 

 improbable that they should arise from the heads of the mushroom- 

 bodies in the Ant, and from a different part of the brain in other 

 insects.* 



Insects' brains vary very considerably as regards the development 

 of the mushroom-bodies. In Ants, Bees, and Wasps they are 

 proportionately large, and double on each side. In the Cockroach 

 they are double, and moderately well developed, and in the Mole- 

 cricket there is said to be only one on each side. Dujardin could not 

 detect these mushroom-bodies in the Diptera ; but recent investiga- 

 tions (E. Berger, " Untersuchungen iiber den Bau des Gehirns und 

 der Retina der Arthropoden." Arbeiten des zoolog. Instituts zu 

 Wein. Bd. I., Heft. II., p. 173) show that certain bodies exist in 

 the Blow-fly (AJusca vomitoria), and the House-fly (31. domestica), 

 which, most probably, are correctly regarded as the homologues of 

 the mushroom-bodies. 



I had already prepared sections of the heads of several insects, 

 some of which have been exhibited at our meetings, before I saw 

 the paper by Dr. Dietl (" Zeitsch. f. wissenschaft. Zool." 1876, 

 vol. 27, p. 488), in which some beautiful sections of Insect brains 

 are figured and described. When I saw them I determined to try the 

 method he had used for hardening the brains, namely, with hyperosmic 

 acid. The insect which I selected to work upon was the Cockroach 

 (Blatta orientcdis). In the first place it was necessary to remove 

 the brain from the head in a perfectly fresh condition, and this 

 required some care, because the organ itself is extremely delicate, and 

 if the investing membrane be injured the internal parts are apt to be 

 squeezed out in the hardening process. The fresh brain, cleared 

 from the surrounding parts, was placed for a few hours (6 or 8) in an 

 aqueous solution of hyperosmic acid (£ to ± per cent.) It was then 



* I find, since this paper was written, that according to Flogel, these 

 nerves pass down beside the mushroom-bodies to the middle of the brain. 

 (" Zeitsch. f. wissenschaft Zool.," 1878, Vol. 30., suppl. p. 556.) 



M 



