At last week's meeting of the Leicestershire Entomological Society someone raised this interesting question. More...
The Common Green Shieldbug (Palomena prasina) is typically green in summer (hence the name). Adults overwinter and turn a bronze colour, turning green again in spring. Adults don't moult, hence the colour change is not due to shedding the exoskeleton - so how do they change colour?
An insect exoskeleton consists of a series of layers - on the outside a waxy, waterproof epicuticle, and inside that a thick chitin-rich procuticle which consists of an outer exocuticle and an inner endocuticle. An insect’s color is derived from a combination of pigmented color and structural color - layers of tiny, reflective plates that cause iridescence - both of these are found in the procuticle layer. At that point my knowledge ran out, so I hit the literature to try to solve the mystery - and didn't really find the answer. But I did find this:
"The strong retardation of nymphal development, smaller size, softer cuticle and lighter yellowish body colour of N. viridula adults as well as their dramatically reduced life span in the 1st August series suggest that the elevated temperature experienced by nymphs and young adults exceeded their thermal optima. The mechanism underlying the abnormality of the cuticle colour and structure is so far unknown, although it might be related not only to thermal stress experienced by the bugs but also to the malfunction of their gut symbiotic bacterial fauna caused by the daily high temperature extremes (T. Fukatsu et al., unpublished data)" (Musolin, 2011).
In other words, it is known that in at least some species of shieldbug, temperature can affect pigmentation. And since adult Palomena prasina do not moult while still managing to change colour, the likely explanation is that the epicuticle must be relatively transparent in this species and the insect can change the pigmentation of the endocuticle, presumably in response to temperature changes.
Now go out there and record some nice bronze-coloured Green Sheildbugs for us!