Introduction to marine insect larvae
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The insects are responsible for the majority of terrestrial biodiversity. In the sea, however, very few insects are found. In Nordic waters marine insects are mostly beetles, Coleoptera, of which both larvae and adults may be associated with brackish waters, and larvae of Diptera, especially the family Chironomidae (Barnes, 1994). The majority of chironomid species have larvae living in freshwater, and only a few species occur in brackish water. In most cases it is not possible to identify dipteran larvae to species. Identification to family level is usually possible, and also to genus in many cases. There are no introduced beetles in Nordic waters, but one species of chironomid larvae is probably introduced and usually associated with man-made structures.
Insect larvae are worm-like and segmented, usually with a distinct head and often specialized structures at the posterior end. Contrary to polychaete (and oligochaete) worms, insect larvae have chitin exoskeletons. They undergo a series of moults, and characters may change for different stages (instars). Beetle larvae have the usual insect complement of 3 pairs of walking legs, whereas dipterans have only prolegs (or parapods) used for locomotion. The propods are not segmented, but stubby wart-like structures. Chironomid larvae are characterized by having two pairs of prolegs, one just behind the head (on first thoracic segment) and the other on the last abdominal segment. The head is usually sclerotized, called the cephalic capsule. Dorsally on the posteriormost abdominal segment are two bundles of setae, anal setae (procercal setae). Some species furthermore have a number of anal tubes on this segment (Epler, 2001). Most species have short antennae, which in one subfamily are retractable. To identify chironomid larvae it is necessary to use a microscope to study head appendages.
Drawing of beetle and chironomid larva A: Beetle larvae - Hydroporus (from Barnes 1994) B: Chironomid larva - Chironomus (from Epler 2010) C: Telmatogeton larva, posterior end - no anal tubules and no procerci (from Epler 2001)
Chironomid larvae are often called "blood-worms" because they contain hemoglobin in their blood. This is seen mostly in species that are adapted to living under hypoxic conditions. Most chironomids are herbivorous grazers, but some are predators. The adult chironomids are known as midges.
Barnes, R.S.K. 1994. The brackish-water fauna of northwestern Europe. Cambridge University Press, Cambridge, U.K., xvi + 287pp.
Epler, J.H. 2001. Identification manual for the larval Chironomidae (Diptera) of North and South Carolina. Introduction.