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When we come to the singular creatures of which Pentastoma and Linguatula are the type, we have the most striking approximation to the worms in external form, but these are induced evidently by their parasitic mode of life. In seeking for the ancestry of our hypothetical Leptus among the worms, we are at best groping in the dark.
At first the head is indicated by the buds forming the rudiments of limbs; the segments to which they are attached do not form a true head until after the mouth-parts have attained their jaw-like characters, and it is not until the insect is about to be hatched, that the head is definitely walled in. We have arrived, then, at our Leptus, with a head bearing two pairs of jaws.
Our own species of the former genus have not been "worked up," or in other words identified and described, so that whether the European T. holosericeum Linn. is our species or not, we cannot tell. Mr. The first is the Leptus? It is only known as yet in the larval or Leptus state, when it is of the form indicated in the cut, and brick red in color.
That insect has been able to maintain its existence, without dwindling like the Leptus into a mere speck, through the great modification in organs and instinct, which adapt it so beautifully to the feathery element in which it moves.
My own observations bearing on this point refer less to the Ixodes than to the minute bete-rouge, which is excessively abundant in the Plata district, where it is known as bicho colorado, and in size and habits resembles the English Leptus autumnalis.
Our only refuge is in the worms, and how to account for the transmutation of any worm with which we are at present acquainted into a form like the Leptus, with its mandibulated mouth and jointed legs, seems at first well nigh impossible.
Its body, not divided into rings, is provided with about thirty pairs of fleshy tubercles, each ending in two strong claws, and the head is adorned with a pair of fleshy tubercles. It is remotely possible that some Silurian land worm, if any such existed, allied to our living Peripatus, may have been the ancestor of a series of types now lost which resulted in an animal resembling the Leptus.
A little water flea was described as a separate genus, Nauplius, before it was known to be the larva of a higher water flea, and so also Leptus was thought to be a mature mite. Accordingly, we follow the usage of certain naturalists in dealing with the Crustacea, and propose for this common primitive larval condition of insects the term Leptus.
The aquatic Protoleptus, as we may term the ancestor of Leptus, may have had such tubes as these, which acted like the swimming bladder of fishes for lightening the body, as suggested by Gegenbaur. It is known that the swimming bladder of fishes becomes developed into the lungs of air-breathing vertebrates and man himself. The next inquiry is as to the origin of the wings.
To such a generalized embryonic form to which the insects may be referred as the descendants, we would give the name of Leptus, as among Crustacea the ancestral form is referred to Nauplius, a larval form of the lower Crustacea, and through which the greater part of the Crabs, Shrimps, Barnacles, water fleas, etc., pass to attain their definite adult condition.
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