Insects
Insects are classified in the Class Insecta, of the Phylum Arthropoda, or
"joint-footed animals". This is a diverse group of organisms which also includes (but is not
limited to) the Arachnids, the Myriapods (centipedes and millipedes),
the Crustaceans and the Pentastomatids, or tongue worms. Insects play an important role as
agents or carriers of disease, as well as acting as ectoparasites. For the purposes of this resource, the insects
have been classified as Insect Vectors or Parasitic
Insects. A third category has been included, which looks briefly at the use of insects in Forensic Entomology.
As the other sections of Wormlearn have demonstrated, many parasites are reliant
on the presence of arthropod vectors or intermediate hosts to continue the life-cycle and to
transmit them to new hosts. Without these vectors, the parasites would quickly become extinct.
This is why any control program for arthropod borne diseases must include measures to limit the
spread of the arthropod vector as well.
Below is a list of parasites and the arthropods which are known to transmit them (a link is
provided if Wormlearn features images of these organisms).
Insect Vectors
| Intestinal Protozoa | Flies (mechanical vectors) |
| Malaria | Anopheles mosquito |
| African Trypanosomes | Glossina spp. - The Tsetse fly (wing only) |
| Trypanosoma cruzi | Triatomid (Kissing) bugs |
| Leishmaniasis | Phlebotomus spp. - Sandflies |
| Wuchereria bancrofti | Mosquitos |
| Brugia malayi | Mosquitos |
| Dirofilaria immitis | Mosquitos |
| Onchocerca volvulus | Simulium spp. - The Blackfly |
| Loa loa | Chrysops (Fly) |
| Dracunculis medinensis | Waterfleas (Crustaceans) |
| Paragonimus westermani | Crabs, Prawns, etc (Crustaceans) |
| Diphyllobothrium, Spirometra | Waterfleas |
| Dipylidium caninum | Ctenocephalides spp. - The Dog and Cat Fleas |
| Hymenolepis spp. | Grain Beetles |
Arthropods may also carry a range of other disease - causing organisms. Mosquitos such as Aedes spp. may carry infections such as Ross River Virus, Dengue and Yellow Fever. Mites and ticks may carry Relapsing Fever, Spotted Fever, Lyme Disease and Scrub Typhus, whilst lice may carry normal Typhus. The Great Bubonic Plagues are sometimes blamed on the rats, while in fact it was the Egyptian rat flea Xenopsylla cheopis (ZEN-op-SY-lah key-OH-piss) which transmitted the plague bacillus to humans.
Very few insects are true parasites. Most live on the surface of our bodies as ectoparasites, feeding on blood or dead skin, but can
survive for short periods of time away from our bodies.
Lice are small wingless insects which are flattened dorso-ventrally (ie. front to back, with
their legs sticking out to both sides). They live on the outer surfaces of their hosts, drinking blood
with specialised mouthparts. It is this feeding activity which generates the itching characteristic
of louse infestation. Lice are extremely host specific - they are limited to which hosts they can
infest by the size of the claws on the ends of their legs. Therefore the human headlouse Pediculis humanus
capitis (ped-ICK- cue-liss hue-MAR-nuss CAP-it-iss) has claws which resemble
the claws on a sandcrab, as these are suited to grasping the fine hairs of the head. The claws on
the pubic louse
Phthirus
pubis (THIGH-russ PEW-biss) are much thicker, like the claws on a mud crab,
for grasping the coarser pubic and armpit hair. Despite the panics which tend to rush through
some school communities, humans cannot catch lice from birds, as the bird lice can only hang
onto feathers.
The two species of louse which infect humans have an interesting history. The closest living
relative to Phthirus pubis is parasitic on gorillas where it lives all over the body, and it
is believed that Phthirus may have been our original parasite, back when the precursors
to humanity had a lot more hair on their bodies. As the human species has become less hirsute,
Phthirus has become confined to the last refuges of thick body hair - the pubis and the
armpits. Pediculis' closest living relative lives on bats, and it is believed that the ancestors
of Pediculis humanus swapped hosts when bats and humans shared caves together. The
claws of Pediculis are unsuited for grasping our coarser body hair, so body lice
(Pediculis humanus corporis) make do with living in our clothing, hopping down onto
the skin to feed.
Body lice are known to transmit Typhus, a disease caused by a type of bacteria known as a
Rickettsia. More commonly, however, lice (both head and body) are an annoyance. Head lice
periodically sweep through schools, perfectly suited for transmission with their cramped quarters.
The head louse never moves more than 1-2cm from the scalp (where it feeds), so cutting hair is
of little value for prevention of infestation (save that shorter hair combs more easily). The lice
like to be able to move through the hair, so they like cleaner, less knotted hair (ie. better cared
for).
The female louse lays oval shaped eggs (or "nits") on the hair, cementing them with a special
glue. If you can remember the scene from Alien, when John Hurt enters the hold of the derelict
ship and finds the field of eggs, that is what the nits look like - a little lid, and just transparent
enough to see something with far too many legs for its own good scuttling around inside. The
total lifecycle, from nit to adult female, takes about 10 days. Many headlouse treatments do not
kill the nits, and so treatment should be repeated within 7-10 days to catch any nits that have
hatched out.
There are a number of headlouse treatments available, and their effectiveness varies
according to the substance used and whether the local population of lice are resistant to them or
not. It is best to cycle through treatments with each outbreak. This puts less selective pressure
on the lice to develop resistance to the treatment. When treating, the whole family must also be
treated, as some people may harbour small populations of the lice without realising it. Afflicted
people should follow strict hygiene measures - no sharing of hats, towels or combs.
Images of Pediculis
capitis and Phthirus pubis
are contained in Wormlearn Fleas are generally less host specific than lice. They are also small
wingless insects, but are laterally flattened (ie. side to side, with the legs all sticking out together
below them). Fleas undergo a metamorphic life-cycle, similar to a butterfly or moth. Eggs are
usually laid on the fur of their host or in the environment close to where the host lives. A
grub-like larva hatches out and feeds on organic matter in the local environment (eg. a dog's
bedding, the carpet, etc). When the grub has finished its development, it forms a cocoon and
transforms into the adult flea. These cocoons may lie dormant in the environment for a long time,
and hatching of the adults is triggered by vibration. This is why we may observe a "swarm" of
fleas attacking us from the carpet when we return from holidays. The adult fleas live on the host,
drinking their blood through specialised mouthparts.
As has been described previously, fleas are important vectors of disease
(eg. dog tapeworm, bubonic plague). Because they are not fussy about which host they leap upon,
the potential for spreading disease is great.
Fleas of medical importance include The Dog and Cat Fleas Ctenocephalides spp. (TEN-oh-KEFF-ah-LIE-dees
KAY-niss), The Egyptian Rat Flea Xenopsylla
cheopis (ZEN-op-SY-lah key-OH-piss) and The Human Flea Pulex irritans
(PEW-lex IH-rih-tans). Another type of flea, the Chigoe Flea, or Tunga penetrans
(TUN-gah PEN-eh-trans) actually burrows into the skin, causing extreme pain)
Images of Ctenocephalides, Xenopsylla
cheopis and Pulex irritans
are contained in Wormlearn The bedbug, Cimex
lectularius (KY-mex LEK-you-LAR-ee-uss) is a small member of the Order
Hemiptera (the True Bugs) which has evolved to live an almost parasitic lifestyle. During the
day, it lurks in cracks and crevices around sleeping areas, emerging at night to feed on the blood
of anyone sleeping nearby. It has lost the wings which are normally found in members of this
order. Bedbugs cause considerable irritation and loss of sleep in afflicted individuals. The
presence of bedbugs in hollow bed heads has been blamed for the unpopularity of these items in
past years.
An image of Cimex
lectularius is contained in Wormlearn When investigators arrive at the scene of a crime, they rely on a number of procedures to
allow them to determine what has transpired. A technique which is gaining in popularity as it is
refined is that of forensic entomology, or the use of insects in crime scene
investigations.
When an animal stops moving, or reacting to the presence of flies around it (such as when
it dies), certain species of these winged insects will deposit eggs in an effort to supply their young
with a ready food supply. Each species of fly has different preferences as to the conditions under
which it lays its eggs. Some flies are only active during the day, others at night. Some prefer to
lay their eggs in shaded areas, while others do it in direct sunlight. Some flies will commence
laying eggs as soon as the animal stops moving, while others wait until the corpse has started to
decay before they lay their eggs.
Therefore, the presence of fly larvae (or maggots) on a carcass depends
largely on the conditions in which the carcass is lying. Similarly, the environmental conditions
will affect the rate at which the fly larvae hatch from eggs and develop to the pupa stage (cold
weather generally slows them down).
By knowing the conditions which a carcass (or human corpse) has been exposed to, and by
carefully comparing the stages of development and species of fly present to tables of
developmental rates, a Forensic Entomologist may gain a rough estimate of the time a corpse has
lain out in the elements, and even whether it has been moved.
But how does one tell one maggot from another ? Surely all maggots look the same.
Thankfully, maggots do have some distinguishing characteristics, such as the shape of their
mouthparts, and the patterns present on their anterior and posterior breathing spiracles. These are
called stigmata
, and are distinctive for different genera (and sometimes species) of fly.
Over longer periods of time, other sorts of insects may be used, such as moths or beetles.
Forensic Entomology is still a growing field, but as more work is performed, and as more
information is gained as to the rates of development of different insects, it is being more widely
used.
An image of maggot stigmata
is contained in Wormlearn, as is an image of the fly
Parasarcophaga crassipalpis. Whilst the latter is of little use in Forensic Entomology,
it's an excellent picture of a flyLice
Fleas
Bedbugs
Forensic Entomology
This page Copyright 1997 Dr Peter Darben.
Last Updated 27.7.2003
