the cases of Sitaris and Meloe were looked on as exceptional ; but in 1862 
the attention of the lecturer was called to the question by observing a 
somewhat similar case in Lonchoptera, a genus of small flies. More- 
over, he found that in many species belonging to the Orthoptera and 
Hemiptera the stages were much less definite and more gradual than had 
hitherto been supposed. 
In illustration of this he described the transformations of Chloeon 
(Ephemeridse), and showed that the perfect form was attained through 
more than twenty changes of skin, each attended by a slight change of 
form. In its preparatory stages this insect lives in the water, but in the last 
two it becomes aerial. Sir John Lubbock had been so fortunate as to see 
more than once the passage from the aquatic to the aerial condition ; 
the larva floated helplessly on the surface of the water, suddenly the 
skin burst, the insect sprang out of the back of its own head and fluttered 
away. The whole process occupied less than ten seconds. 
The speaker in this case wished particularly to impress on his 
hearers, firstly, the gradual nature of the changes, and secondly, that 
some of them have no reference to the form of the perfect insect, but 
are entirely of an adaptational character. Thus, the young larva is 
born without branchiae, and with two caudal appendages. It gradu- 
ally acquires a thin tail and seven pairs of branchiae, but the perfect 
insect has only two tails and no branchiae. Thus, then, the changes 
which an insect undergoes are of two kinds, developmental and adap- 
tational. 
External forces act upon the larvse as much as on the perfect 
insects. And we can thus understand the remarkable fact that some 
animals, which differ much when young, are very similar at maturity. 
The speaker then entered into some theoretical considerations as 
to the nature and causes of metamorphoses, dividing the subject into 
three questions. 
Istly. How these changes of form might have originated. 
2ndly. Why they are, in insects, so abrupt in their character ; and 
3rdly. Why the pupa condition, a period of approximate immobility, 
should intervene between the active larva, and the still more active 
imago. 
1. The changes of form depend on the early condition at which 
some insects quit the egg. There is reason to believe that all insects 
pass through the stage of fat, fleshy grubs, and subsequently acquire 
legs.* Some, however, are hatched in the first state, while others 
remain in the egg until they attain the second. In the former case 
additional changes are produced by the fact that external forces do not 
affect the larva in the same manner as the perfect insect ; and thus 
there is a tendency to still greater differentiation. 
2. The abruptness of the change is more apparent than real. The 
actual change itself is merely the withdrawal of the curtain, the casting 
* Sec, for instance, Professor Huxley's admii-ablo memoir on Aphis, in the 
' Linnean Transactions.' 
