ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 41 



Chordotonal Sense-organs and the Hearing of Insects.* — In a 

 long and elaborate account of this subject, including numerous fresh 

 observations, Prof. V. Graber describes under the above new designa- 

 tion the rod-like terminal secretory structures of the nerves of certain 

 parts (chiefly legs or wings) of insects. The general type of rod 

 is distinguished as Sculopal, or pencil-like, being pointed at the 

 proximal end; this form is always hollow, and its walls are extra- 

 ordinarily refractive. In general, an insect has but one form of 

 these rods. Two subordinate forms are distinguished : (a) Mono- 

 nematic and (/?) ampMnematic, according as the distal end is, or is not, 

 pointed like the j^roximal ; in the mononematic the distal end runs 

 out into a slender filament. Mononematic rods may be either (a) 

 conocephalic, with conical heads (larva of Tahanus, of Tortrix sp., 

 Orthoptera and some Formicidce) ; (b) A'piocephalic (a Phryganid- 

 larva), head blunter; (c) Conacocephalic, truncate-conically headed 

 (Dytisciis, some Chironomus larvje, &c.) ; {d) Cylindrocephalic, head of 

 equal diameter throughout (a saw-fly larva). 



The amphinematic form of rod occurs in Corethra, larva of SyrpTius, 

 PedicuUdce. The fine distal process is to be regarded as the termina- 

 tion of the head of the rod, and not as a prolongation of the nerve- 

 fibre. An essential difference between the amphinematic and mono- 

 nematic rod is that, in the former, the nervous axial filament is firmly 

 fixed or stretched within the cavity of the rod, while in the latter its 

 distal end lies loose in the liquid which this cavity contains. 



In the "scolopophors," or tubular end -organs of the chordotonal 

 nerves, the " chord " of the rod is a prolongation of an axial process 

 of the basal ganglion-cell. The usually compound masses of these 

 organs vary greatly in the number of units contained in them ; from 

 occurring singly in Tahanus most Chironomi, they may number 

 upwards of 100 (tympanal organ of Acrididce) or 200 (some " pori- 

 ferous " organs) in the same sense-organ ; where they are few in 

 number they are commonly very intimately connected, so that in some 

 cases the contours of the different tubes are almost invisible (Corethra, 

 Ptychoptera, Tortrix, SyrpJms, &g.). In some genera (GoretTira, &c.) 

 the chordotonal organ of each segment is fastened to the integument 

 by a special " ligament " consisting of a thin-walled tube in continua- 

 tion with the sheath of the nerve, and filled with a homogeneous and 

 slightly granular mass, and extending in a direction opposite to that 

 of the chordotonal organ itself. 



Of the general positions in which these organs occur, Graber states 

 that the typical forms are always extended from one relatively immo- 

 bile point of the integument to another; e. g. any one of the organs is 

 wholly contained in one segment, and never invades the bands con- 

 necting the segments ; they also show a tendency to have as great a 

 length as the available space will admit of, and to maintain a rela- 

 tively superficial position. They are as widely distributed among 

 insects as the optic and tactile organs, in proof of which tables are 

 given, deduced from observations (chiefly by Graber) made on upwards 



* Arch. f. Mikr. Anat., xx. (1882) pp. 506-640 (6 pis.). 



