Fig. 186. Embryo of Hydrophilus piceus viewed from the ventral surface.
(After Kowalevsky.)
pc.l. procephalic lobe.
The appendages arise as paired pouch-like outgrowths of the epiblast and mesoblast; and their number and the order of their appearance are subject to considerable variation, the meaning of which is not yet clear. As a rule they arise subsequently to the segmentation of the parts of the body to which they belong. There is always formed one pair of appendages which spring from the lateral lobes of the procephalic region, or from the boundary line between these and the median ventral part of this region. These appendages are the antennæ. They have in the embryo a distinctly ventral position as compared to that which they have in the adult.
In the median ventral part of the procephalic region there arises the labrum ([fig. 187], ls). It is formed by the coalescence of a pair of prominences very similar to true appendages, though it is probable that they have not this value[172].
Fig. 187. Two stages in the development of Hydrophilus piceus. (From Gegenbaur, after Kowalevsky.)
ls. labrum; at. antenna; md. mandible; mx. maxilla I.; li. maxilla II.; p´ p´´ p´´´. feet; a. anus.
The antennæ themselves can hardly be considered to have the same morphological value as the succeeding appendages. They are rather equivalent to paired processes of the præ-oral lobes of the Chætopoda.
From the first three post-oral segments there grow out the mandibles and two pairs of maxillæ, and from the three following segments the three pairs of thoracic appendages. In many Insects (cf. Hydrophilus) a certain number of appendages of the same nature as the anterior ones are visible in the embryo on the abdominal segments, a fact which shews that Insects are descended from ancestors with more than three pairs of ambulatory appendages.