Aesculus hippocastanum, Nieuwendam; head, dorsal and ventral, of the sap drinking stage
The very first larval instar is rather squat.
The next stage, dorsal…
… and ventral.
The larvae are described by Sefrová & Skuhravy (2000a). As in all Gracillariidae the demonstrate hypermetamorphosis, but in a way that fundamentally differs from that of the related genus, Phyllonorycter.
There are six larval instars (Sefrová & Skuhravy, 2000a). During the first four instars (pictures above) the larva’s only food is cell sap. The structure of head and mouthparts is completely adapted to this diet. Of the mouthparts only the mandibles remain. They resemble a pair of garden shears, held horizontally, but with strongly widened blades that moreover have got a serrated front edge. The plant’s cell walls are simply slit open. The head tapers to the front, and has a strongly enlarged upper and lower lip, preventing the leaking away of the cell sap.
The larvae of these four instars have no feet. The is strongly flattened, as an adaptation to the very shallow mine in which the larva lives. All segments are strongly widened laterally, enabling the larva to brace itself despite the lack of feet. The head is almost colourless; all segments have transverse shields, brown dorsally, almost colourless ventrally. The larvae in these instars lack silk glands.
In connection with the diet the frass initially is a semi-liquid mass, that dries to a tar-like substance. Remarkably, the frass is not deposited to the ceiling, but rather the floor of the mine, and concentrated in its centre.
larva of the chewing stages, dorsal …
… and ventral.
the thoracic feet
The last two instars look totally different. They are much more strongly pigmented. There are thoracic feet and prolegs, even though the thoracic fee are not very prominent. But especially the head and mouthparts are strongly altered. The upper and lower lips are inconspicuous now, and small.
In Phyllonorycter a similar deep change in morphology takes place (only the later stages are rather lighter, instead of darker). The the later stages have strong, triangular mandibles, capable of chewing. In connection with the cellulose-rich food from now on the frass in Phyllonorycter consists of dry granules. In Cameraria the situation is very different: there are mandibles indeed, but they consist of small fragments; probably they are totally functionless. These two larval instars do not play any role in food intake (and obviously there is no change in frass type). The two instars seemingly have no biological function, and are of quite short duration. Yet there is one single function they do fulfil: they have silk glands, and this stage can make a cocoon. Most of the time the cocoon does not mean much, but for pupae about to hibernate this is different.
Because their brief life, larvae of the chewing type are somewhat difficult to find. Much more frequently their exuvia are seen.