The Characteristics of the Insect Order Siphonaptera

Siphonaptera are a small order of highly specialised insects known as fleas.  There are over 2000 species of fleas worldwide, most of which are ectoparasites of mammals, with about 100 species found on birds.  The scientific name, Siphonaptera, refers to the tube (siphon) that is used to suck blood and the fact that fleas are wingless (aptera is Greek for without wings).

Fleas are well adapted for their chosen lifestyle.  Their bodies are laterally flattened and covered by backward-facing hairs and spines, sometimes arranged in combs, which help them to move easily through fur or feathers.  They are wingless and instead use their strong legs for climbing and jumping and have large claws for gripping on to the host’s fur.  Their mouthparts allow them to pierce the skin of their host and suck up blood.  Unlike many insects, fleas have no mandibles.  Instead they have blade-like maxillae for cutting and inside the epipharynx is a long stylet and a pharyngeal pump for extracting blood.

The flea’s head has a pair of three-segmented antennae, which are longer in the male, who uses them as a secondary set of claspers during copulation.  Fleas have no compound eyes and the simple ocelli are often vestigial or absent so sight is not an important sense for these animals.  Behind the head is a large, muscular thorax which powers the legs and behind that is the 8 segmented abdomen.  All three body parts are laterally flattened and covered in backward-facing hairs.  The abdomen contains a large stomach, capable of holding a big blood meal.

Fleas have a complete life cycle, starting with small, whitish, oval eggs of about 0.5 mm long which hatch after 2- 12 days into whitish, worm-like larvae.  The larvae have 13 hairy body segments, short antennae, no eyes and are about 4-10 mm long.  They are not parasitic and are usually found living in the host species’ nests or bedding, where the larvae feed on dead skin and other organic matter.  After anywhere from 10 to 200 days, depending on temperature, humidity and food availability, the larvae form pupae.  In the pupal case, the developing adult flea goes through a stage where wing buds are formed.  These show that fleas are descended from winged ancestors and only secondarily flightless.  The pupal stage can also last either a long or short time depending on environmental conditions.

When the young adult fleas emerge from the pupal case, they go looking for a host.  This may be easy if they are in a nest or they may have to hop around until they find a suitable host.  Fleas in general are not as host dependent or host specific as lice, the other major insect ectoparasites.  Some flea species have been found on 20 or more hosts species and some hosts have more than one attendant flea species.  A few, such as the echidna flea, are host specific and found nowhere else.  Fleas quickly abandon a dead host and are not usually fussy about the species of the new host, which is why fleas are so good at spreading the plague from rats to humans.

This is the main economic significance of fleas: they are vectors for a number of diseases as well as the plague.  Their bites are annoying but it is the other parasites that pass from the flea’s saliva to our blood that makes them potentially lethal.  They are also intermediate hosts for tapeworms.  It has been suggested that rats and fleas have caused more human deaths than wars and as such have influenced the course of human history on many occasions.

Sources:  Britton etal 1979  Insects of Australia: A Textbook for Students and Research Workers   CSIRO Australia