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The external differences between the larval and pupal states are fixed for a large part of the year in most butterflies and moths, though even in this respect there is every possible variation, some moths or butterflies passing through their transformations in a few weeks, others requiring several months, while still others take a year, the majority of the moths living under ground in the pupa state for eight or nine months.
This is not so, for like the adult fish, the larval tadpole, and the embryo chick, an embryo of a cat or a man is at one time constructed with a series of gill-clefts and with blood-vessels and skeletal supports of fishlike nature that are everywhere associated with gills.
This naked journey over the leaves, the last joy of its larval life, is fatal to the traveller. Up comes the Tachina, who, finding a clean skin, all sleek with fat, loses no time in dabbing her eggs upon it. A census of the intact and of the injured larvæ provides us with particulars which agree with what we foresaw from the nature of their respective lives.
It is the same with every change which gives to the larval stages, rendered multifarious by crossed and oblique characters, a more straightforward direction, simplifies and abridges the process of development, and forces it back to an earlier period of life, and finally into the life of the egg.
Again, with respect to the singular fact that many terrestrial and fresh-water animals do not undergo any metamorphosis, while marine members of the same groups pass through various transformations, Fritz Muller has suggested that the process of slowly modifying and adapting an animal to live on the land or in fresh water, instead of in the sea, would be greatly simplified by its not passing through any larval stage; for it is not probable that places well adapted for both the larval and mature stages, under such new and greatly changed habits of life, would commonly be found unoccupied or ill-occupied by other organisms.
These are all formed on the same plan, arising early in the larval stage as three pairs of little tubercles, which ultimately form long blades, the innermost constituting the true ovipositor. We have found that one pair of these organs forms the "spring" of the Podura, and that in these insects it is three jointed, and thus is morphologically a pair of legs soldered together at their base.
I will describe this double stage of the organism by the phrase 'larval dimorphism. The initial form, that issuing from the egg, I will call 'the primary larva; the second form shall be 'the secondary larva. Among the Anthrax flies, the function of the primary larva is to reach the provisions, on which the mother is unable to lay her egg.
Thus in his larval life does he symbolize some restless nation that makes itself many successive constitutions and forms of government, in none of which it abides long; but afterwards some higher thing, when he rests motionless, in form like a sarcophagus, whence the infolded life emerges to haunt the twilight a grey ghost moth.
All of the efforts directed against the adult mosquitoes are usually of little avail in decreasing the number in any region. It is comparatively easy, however, to fight them successfully in the larval stage. We have seen that standing water is absolutely necessary for mosquitoes to breed in.
There are several similar cases among Reptiles and Amphibia. In these also rudiments of limbs are entirely absent in the embryos or larval stages.
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