ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 47 



In the earlier stages of the larvse the nervous system turns con- 

 siderably out of the direct line, in order to allow the duct of the gland 

 to pass ; but, in the full-grown or nearly full-grown larvfe, it is nearly 

 straight, although still distinctly unsymmetrical. This lessening of 

 asymmetry, as the larva grows, is due to the duct being somewhat 

 smaller in larger larvae, in proportion to the size of the larva, thus 

 allowing the nervous system to settle back, more or less, into its 

 normal position. 



This kind of asymmetry has not been found, the author believes, 

 in any other Arthropod, but, upon the suggestion of Prof. Leuckart, 

 she examined Hirudo mecUcinalis, the blood-leech, the nervous system 

 of which has an analogous asymmetry. The genital organs are in 

 such a position as to necessitate the pushing of the nervous system 

 slightly to one side, near their outlet. Of four specimens of Eirudo 

 examined, two had the nervous system to the right and two to the left 

 of the genital organs ; but of six specimens of Harpyia dissected, all 

 had the commissure between tbe first and second thoracic ganglia 

 deflected toward the left. 



Vitality of Insects in Gases.* — From the apparent indifference 

 of some insects to foul and poisonous emanations, as well as the vary- 

 ing sensitiveness of others under similar conditions, it would seem 

 reasonable to conclude that there is a substantial difference in the 

 delicacy of their respiratory functions, which might be indicated 

 approximately by subjecting individuals of various groups to artificial 

 atmospheres of deleterious or irrespirable gases. This opens a wide 

 field of experimentation both in the methods employed, the reagents 

 used, and the insects examined. More from curiosity than any other 

 motive, Mr. L. P. Gratacap has made some trials in this direction, the 

 results of which he tabulates, though they have not been extended 

 enougb to admit of any very interesting deductions. 



The gases used were oxygen, hydrogen, carbonic oxide, carbonic 

 acid anhydride, prussic acid vapours, nitrous acid fumes, chlorine, 

 laughing gas (nitrous oxide), illuminating gas, and ammonia. 



In oxygen the insects at first showed slight symptoms of exhila- 

 ration, "accompanied with a restless inclination to jump"; but this 

 passed away, and they seemed totally unaffected by the excess of 

 oxygen, and their vitality was impaired only after long exposure to its 

 influence, due in some cases as much to the confinement. Flies (Musca 

 domestica) lived 29 hours ; Colorado beetles and meal bugs were con- 

 fined 3 days, and then revived completely ; Phalangium dorsatum 

 lived 24 hours; Noctua 1\ day; and the common yellow butterfly 

 (Colias philodoce) died in 12 hours, possibly as much from the effects 

 of its own violence as from the gas. In hydrogen the flies were at 

 once paralysed, and though apparently dead, were alive " for a long 

 time " afterwards. Noctua and a black wasp died at once, but Colorado 

 beetles evinced wonderful vitality, and revived thoroughly after 

 almost 2 days' immersion. In carbonic acid anhydride, flies at 

 once died, while Colorado beetles recovered after 3 hours' exposure, 



* Amer. Natural., xvi. (1882) pp. 1019-22, 



