Mottled bee-fly  (Thyridanthrax fenestratus)

Abdomen: in arthropods (crustaceans, insects and arachnids) the abdomen is the hind region of the body, which is usually segmented to a degree. In crustacea (e.g. crabs) the limbs attach to the abdomen; in insects the limbs are attached to the thorax (the part of the body nearest to the head) and not the abdomen. In vertebrates the abdomen is the part of the body that contains the internal organs (except the heart and lungs).
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.
Parasitoid: an insect that lives in or on a host that is also a parasite on another species. If the parasite kills the host it is commonly called a parasitoid.
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 (August 2002)
   http://www.nhm.ac.uk/nbn
   
2. UKBAP (August 2002):
   http://www.ukbap.org.uk
   
3. From observation of images.
   
4. Allaby, M. (1991) The concise Oxford dictionary of Zoology. Oxford University Press, Oxford.
   
5. Jon Webb (2002) Pers. Comm.