Another peculiarity connected with the present head is the augmentation in bulk which takes place, after exclusion, in the eggs of the great tribe of saw-flies (_Tenthredo_ L.), the gall-flies (_Cynips_ L.), the ants (_Formica_ L.) and the water-mites (_Hydrachna_ Maïll. _Atax_ F.). Those of the two former, which are usually deposited in the parenchymous substance of the leaves, or of the young twigs, of various plants, imbibe nutriment in some unknown manner, through their membranous skins, from the vegetable juices which surround them[159], and when they have attained their full size are nearly twice as large as when first laid. Except in the eggs of fishes, whose volume in like manner is said to augment previously to the extrusion of the young, there is nothing analogous to this singular fact in any other of the oviparous tribes of animals, the eggs of which have always attained their full size when they are laid.
It is to M. P. Huber that we are indebted for the knowledge of the fact that the eggs of _ants_ grow after being laid, a circumstance favoured probably by the moist situation in which the workers are always careful to keep them. By an accurate admeasurement he found that those nearly ready to be hatched were almost twice as big as those just laid[160]. A similar observation was made on the red eggs of a water-mite (_Hydrachna abstergens_) by Rösel, who conjectured that they draw their means of increase from the body of the water-scorpions (_Nepæ_), of which they form so singular an appendage[161], which opinion is confirmed by De Geer, who observes that when the water-scorpions are covered by an unusual number of the eggs of the water-mites, they grow weak and languid, and endeavour to rid themselves of their parasitic appendages[162]. It is most probable that the mite lately named (_Uropoda vegetans_), which is often found planted as it were upon the bodies of various beetles, by means of a long pedicle, through which, as the foetus by an umbilical chord and placenta, it derives its nutriment from the above animals, is at first so fixed in the egg state, though before it is disengaged from the pedicle it is hatched, since it is often found with its legs displayed and quite active--this is the more probable, as the eggs of the water-mite are fixed by a pedicle to the animals to which they are attached[163]. I have met with a remarkable instance, in which pedunculated eggs seem to draw nutriment from the _mother_, which brings the pedicle still near to the nature of the umbilical chord. Those of the small hemipterous insect which infests the larch before alluded to, are attached to the anal end of the mother by a short foot-stalk not longer than the egg.