Great diving beetle  (Dytiscus marginalis)

Elytra: in beetles and earwigs, the hard fore wings. They are held aloft when the insect flies, and are often coloured or patterned.
Larvae: stage in an animal's lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Thorax: part of the body located near the head in animals. In insects, the three segments between the head and the abdomen, each of which has a pair of legs.

1. National Biodiversity Network Species Dictionary (Jan 2003):
   http://www.nhm.ac.uk/nbn
   
2. Harde, K. W. (2000) A field guide in colour to beetles. Silverdale Books, Leicester.
   
3. Lyneborg, L. (1976) Beetles in colour. Blandford Press, Dorset.
   
4. Joy, N. (1933) British beetles; their homes and habits. Frederick Warne & Co., Ltd., London.
   
5. Animal Diversity (March 2003):
   http://animaldiversity.ummz.umich.edu/accounts/dytiscus/d._marginalis$narrative.html
   
6. Buczacki, S. (2002) Fauna Britannica. Hamlyn, London.