Whilst looking for galls on oaks last month I was lucky enough to come across this scene, some insects hatching out. Quite simple, but there is actually a lot going on here.
First, what species is this? Looking at the number of antennal segments, the sucking mouthparts and the shape of the insects I’d say that we are looking at Hemiptera, the true bugs, sub-order Heteroptera. These all have incomplete metamorphosis, with the wingless juveniles (nymphs) looking increasing like the adults as they develop and moult, unlike insects such as butterflies which have complete metamorphosis with a larval stage changing through a quiescent pupal stage into a radically different adult stage. We can narrow down the species of these bugs further: their rotund shape suggests they are probably a member of one of the families of shield bugs. The egg shape (barrel-like), colour (whitish-green), position of laying (in a batch of about 20 on the underside of an oak, Quercus robur, leaf) and that the larvae are emerging through a ‘lid’ (the operculum) in the top of the egg (look at the one emerging at the bottom right of the egg batch), rather than splitting the egg longitudinally down the middle, all narrow the species down. So, I think we have here the green shield bug, Palomena prasina, a common member of the Pentatomidae, which in time will produce large green adults.
Second, what is going on here? There are some eggs which may not have hatched yet, and there seem to be two sorts of nymphs, small blackish ones and larger yellowish ones. The blackish nymphs are the first instar (instar is the term for the stage in between moulting, so first instars have just hatched and have not moulted again). Like many Hemiptera this species is a plant feeder, and needs the help of symbiotic gut bacteria in order to process the plant phloem it sucks up. These bacteria are transmitted from generation to generation in shield bugs by being smeared by the female over the eggs she has laid. The first instar larvae are busily sucking up these bacteria from the egg shells. Indeed, this is the only feeding they do in the first instar, and they moult within a couple of days into the second instar. The second instars are the larger paler individuals – look at the one on the right, it has just emerged from its old skin, the blackened and shrivelled remains of which can be seen to its rear (some more skins can be seen around the other larvae). As they mature, these second instars will turn greenish, and will disperse to forage independently. They go through five larval instars in all before moulting into an adult, after about 6 weeks.