A HISTORY OF SURREY markings, may sometimes have the green replaced by bright mulberry ; this is a very pretty variety. It is not known if the descendants of a mulberry coloured larva will also be mulberry or whether it is merely a ' sport,' but the two forms may be found feeding side by side. Cimbex lutea and Nemafus turgidus also have occasional red forms ; these abnor- mally coloured larva? are very delicate and difficult to rear. Many species have brightly coloured larva? which feed in company and are very conspicuous. They protect themselves from birds by their abdominal glands which emit a powerful and disagreeable odour and (probably) by a disagreeable taste. The Croesus genus furnish good examples of this peculiarity, especially C. septentrionalis, which gives out a disgusting and fetid smell when disturbed. The Dineura genus, notably D. stilata, are protected in the same manner. Nemafus abdominalis on the other hand, an inconspicuous green larva, is very fragrant, its scent resem- bling that of the lemon verbena. Solitary feeding larvas generally trust to protective colouring for their safeguard and are often very difficult to detect even when they are known to be on a given plant. Such is Camponiscus luridrventris, an absolutely flat green larva which lies stretched out on the undersides of alder leaves on which it feeds. It resembles some kind of scale insect rather than a caterpillar. Others imitate bird droppings, for instance Eriocampa ovata, which is covered with a white flaky substance disguising it very effectually. The flakes can easily be rubbed off and disclose a pale green larva, but they are renewed and intensified after each moult. All these devices however effectual against birds seem powerless to repel the attacks of ichneumons and parasitic Diptera to which sawflies are exceedingly subject. Every species suffers from one or more of these parasites, and it is chiefly by their agency that in this country sawflies do not often multiply sufficiently to do serious damage, as has sometimes happened in Germany where swarms of Lopbyrus pint have destroyed whole pine plantations. Gallflies form a very large group of insects and much still remains to be discovered concerning their economy. Wherever oaks and roses thrive there will the gall makers and their inquilines and parasites abound. Galls vary infinitely in appearance, texture, structure and position ; they may be situated on any part of a plant including the roots, may contain one or many inhabitants, may drop off the plant in autumn or remain growing with the twig long after the fly has escaped. They may be smooth, spiny or warty ; green, white, brown, yellow, pink or purple ; regular or irregular, round or spindle shaped, globular or ovoid ; woody, succulent or stringy. Mr. Cameron in his Monograph, after mentioning the theories held by different authorities on the origin of galls and how they are produced, concludes that as regards the Cynipida ' there is no evidence that the venom emitted by the insect when laying has anything to do with the origin of the gall, and that as observation shows that the mechanical irritation produced by the birth and growth of the larva is the primary factor in gall genesis, we may fairly conclude that the theory 92
Page:VCH Surrey 1.djvu/134
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