OF THE TRACHE.E IN INSECTS. 29 



which enters, over the carbonic acid which escapes, is in all cases so necessary to expand 

 the tracheae as Dr. Graham suggests, because I have found the small branchlets in a 

 larva of Iftisca still full of air after it had been drowned by immersion for some houi's in 

 water. 



On putting some larvae of Melolontha into water, I was surprised to see a considerable 

 formation of bubbles on the skin, especially at a point below each spiracle, while no 

 bubble ever appeared from the sjiiracles themselves. If, however, the water is first boiled, 

 then no bubbles are produced, and the larvae very soon, say in about a quarter of an hour, 

 become motionless, though, if pricked, they still contract a little. Their flesh is then 

 quite soft and flabby, while generally it is tolerably firm to the touch. I expected to 

 have found the tracheae free from air, or nearly so ; but this was not the case. Like 

 other insects, these larvae readily recover from their suffocation when they are taken out 

 of the water. 



The larvae of flies are also naked fleshy grubs ; and I expected them to behave in a 

 similar manner; this, however, is by no means the case. They live much longer in 

 water. When they are placed in it, no bubbles form on their skin ; nor does it seem to 

 make any difference to them whether the water is boiled or not : I put four into some 

 boiled water, and the same number into water' which had not been boiled ; and at the end 

 of forty-four hours they still moved a little, gently turning their heads from one side to 

 the other. These facts seem to me to prove that the larvae of Melolontha breathe partly 

 by means of their skin, and that those of Musca do not. 



Yet these opposite states of the skin may be necessary for these larvae, living as they 

 do under such different circumstances *. 



This result was quite unexpected by me ; yet it throws much light on the intermediate 

 stages which, upon the principle of natural selection, must have existed between ordinary 

 larvae, respiring principally through spiracles, and those which, like the larvae of Botys, of 

 Dragonflies, Ephemerae, &c., breathe by means of foliaceous expansions of the skin. 



In most insects the air will be found, after death, filling the fine ends of the tracheae. 

 In some cases, however, as in many parts of Carabus, Melolontha, Acheta, Hiiyparchia, &c., 

 the smaller branchlets are generally, even very soon after death, filled with fluid, and can 

 therefore scarcely be distinguished, or they even become quite invisible. This happens very 

 frequently in Lepidoptera, Coleoptera, and Orthoptera ; but I have not noticed it so often 

 in Hymenoptera, Diptera, or Neuroptera. It is not, however, constant in the first three 

 orders ; and in Neerophorus, for example, the very fine tubxiles may be beautifully seen. 

 The larvae, at least of Coleoptera and Lepidoptera, do not in tliis respect resemble the per- 

 fect insect ; but in Acheta all the three forms are alike. 



* M. Lyonet, writing before the observations of Dr. Graham had thrown so much light upon the subject, and mis- 

 led principally by the absence of special respiratory movements in many insects, doubted whether the tracheae were 

 organs of respiration, and suggested that one at least of their uses might be " de coucourir avee les nerfs, a la contrac- 

 tion des muscles, pour operer les mouvements," though I confess that I do not quite understand in what way the 

 tracheae were to cooperate with the nerves. In support of this view, he mentions that if the spiracles are closed by 

 oil, or if the insect is kept under water, after a while it loses all power of movement. This experiment, however, 

 hardly justifies the conclusion which M. Lyonet deduces from it ; and it seems to me that the larvae lose their power 

 of movement in the same way as M. Lyonet himself would, had he been treated in a similar manner. 



