TRANSACTIONS OF THE SECTIONS. 89 



quadrate ; but that it is far more probable that a subdivision of the temporal takes 

 place, corresponding with the subdivision of the lower jaw, and of certain other 

 tones in ovipara, and that the glenoid part is represented by the quadrate, and the 

 zygomatic part by the quadrat ojugal. This view is, moreover, in accordance with 

 the relations of muscles and nerves to the respective parts, and with the position 

 of the quadrate and the glenoid. 



An Examination of the British Lepidoptera, with a view to investigate the 

 Origin of Species. By Dr. Jokdan. 



i: 



On the Transformations of Chlocon (Ephemera) dimidiatum. 

 By Sir John Lubbock, Bart.,F R.H., V.P. Linn.Soc. 



The author commenced by observing that Chloeon dimidiatum, the subject of his 

 aper, belongs to a group of insects (the Ephemeridae), which has become a house- 

 lold word among us, as the type of a short life. The idea holds good, however, 

 only if we consider exclusively the life of the animal in its perfect condition ; but 

 if we regard the whole duration of life from the time of leaving the egg- until the 

 moment of death, it is by no means so appropriate. 



The larva, in the earliest stage observed by the author, is a minute, transparent, 

 active creature, with a large head, a tapering abdomen, and two long- caudal appen- 

 dages. It spends about a year in the water, during which time it increases in size, 

 and alters considerably in appearance. These changes, however, are produced quite 

 gradually, the insect going through about twenty changes of form, each accompanied 

 by a change of skin. 



The antennae increase in length and in the number of segments at each moult, 

 and it is remarkable that this increase is not produced by a growth of the en (ire 

 organ, but by a rapid development and division of the third segment, counting 

 from the base. In the first stage the larva has no respiratory organs, either exter- 

 nal or internal. After the first change of skin, however, the posterior angles of 

 several abdominal segments become elongated, and after one or two more moults, 

 these elongations have taken the form of the gills, or branchiae, characteristic of 

 the species. At the same time, the tracheae make their appearance, i^o far as the 

 author is aware, no other insect has yet been observed which is entirely destitute 

 of trachea?. After the first one or two moults, a minute knob appears between the 

 two caudal appendages, and with each moult this knob increases in length, so that 

 the larva which had originally two tails, finally possesses three. 



After about eight moults have taken place, it maybe observed that the posterior 

 mesothoracic angles are slightly elongated. At each change of skin, these, the first 

 rudiments of the wings, become more and more apparent, and in the last aematic 

 stage these rudimentary wing-cases cover the first two or three abdominal segments. 



The external sexual differences commence to manifest themselves at the eigh- 

 teenth stage. They consist principally of the rudiments of the pillared eyes, and 

 of the processes on the under side of the penultimate abdominal segment, both of 

 which are characteristic of the male sex. 



The author then described the manner in which the larva quits the water, a pro- 

 cess which does not occupy more than ten seconds. It is well known that the 

 ephemeras pass through one more moult after acquiring their wings and leaving the 

 water. The author described the principal differences between the pro-imago and 

 the imago, or perfect insect, and concluded by some general observations on the 

 subject of insect metamorphoses, laying special stress on the fact that, as was shown 

 in this paper, the changes in the larva are not all in the direction, so to say, of the 

 perfect form. Thus, for instance, the young larva has two tails; a third is gradu- 

 ally developed ; but again the perfect insect has only the original two. 



If we bear this in mind, and remember that external circumstances continually 

 act upon the larva?, that the form and characters of the larva have special reference 

 to its own habits and conditions of life, we shall throw some light on the otherwise 

 inexplicable fact that in some cases insects and other animals which agree very closely 

 in their perfect condition have larva? which differ a good deal from one another. 



