This article explains the basics of insect biology for those that are not familiar with it.

Anatomy of an typical insect (fly). Copyright P. Junquera

Insects are one of the groups of animals with the highest number of species. More than one million species are currently known, but it is estimated that there are still millions of species yet to be discovered.

Insects can be as small as a few millimeters (e.g. lice, gnats) and as big as over 10 cm (e.g. some cockroaches, crickets and beetles.).

Insects belong to the group of animals called arthropods (meaning "articulated feet") together with ticks, mites, spiders, crustaceans (such as crabs and lobsters) and others (e.g. millipedes).

For specific information on the various parasitic insects select an option from the corresponding menu, or select one in the list at the end of this article.

Anatomy of insects

The body of most insects is divided in three different parts: head, thorax (meaning breast) and abdomen (meaning belly).

The head has complex eyes, a pair of antennae and various mouthparts. According to the way it feeds, these mouthparts are adapted for piercing (e.g. biting blood-sucking flies, fleas, mosquitoes, etc.), for sucking (many other flies, butterflies, etc.) or for chewing (e.g. crickets, wasps, etc.). The two antennae are movable sense organs usually in the front of the head, mainly olfactory or to detect movements.

The thorax has 3 segments, each one with a pair of legs. The wings are inserted in the first and the second segments. Some insects have two pair of wings (e.g. wasps, bees, butterflies, etc.), other insects have only one pair of wings (e.g. house flies, stable flies), and others don't have wings at all (e.g. lice, cockroaches, etc.).

The abdomen has a variable number og segments, but neither legs nor wings. It contains the reproductive organs, as well as parts of the digestive and excretory systems.

The body of insects is covered by a hard "skin", a kind of shell called the exoskeleton or outer skeleton. It protects the body organs against mechanical pressure, desiccation and external pathogens, and offers attachment sites for the muscles. The exoskeleton is formed of various structures, one of which is the cuticle that contains chitin. Chitin is a special molecule responsible for the hardness of the cuticle that is specific to arthropods, but is also found in some fungi and bacteria.

Insects are highy complex organisms despite their small size. They have digestive, nervous, excretory and respiratory systems. Instead of blood they have hemolymph that fills the main body cavity (called hemocoel) between the organs.

Biology of insects: metamorphosis and life cycle

Life cycle of a house fly. Copyright P. Junquera

All insects have a so-called indirect development: they undergo a process called metamorphosis (a word of Greek origin meaning "form change"): they change their form several times along their development, which includes several stages: egg, larva, nymph or pupa, and adult (also called imago). 

Some insects undergo a complete metamorphosis (they are called holometabolic), e.g. flies, beetles, butterflies, etc. Adult females lay eggs from which small larvae hatch that look like small worms that crawl around to find food. Larvae grow through a series of moults until they become mature (so-called stage-III larvae). Such mature larvae stop crawling and become pupae (the chrysalid of butterflies) that don't move. Inside the pupa the adult is formed and hatches out after several days. Adults don't moult and they don't grow anymore, among other reasons because the rigid exokeleton makes it impossible.

Other insects undergo an incomplete metamorphosis (they are called hemimetabolic), e.g. cockroaches, lice, crickets, etc. Larvae don't look like small worms, but like small adults. These larvae moult several times and increase their size until they become nymphs. They don't go through the pupal stage.  Nymphs look like smaller adults too, but they cannot reproduce. The nymphs moult a last time to become adults without going through the pupal stage.

Metamorphosis is regulated through two major hormones, ecdysone and juvenile hormone

Depending on the species, the whole life cycle may take less than a week or more than a year to be completed. Within each species, the duration of the life cycle strongly depends on environmental factors such as temperature and humidity as well as on the availability of food. Duration of the life cycle is a very important factor to be considered when establishing strategies for controlling a given parasite.

In some insect species the whole life cycle is completed on the host (e.g. lice). In many other species only adults or larvae are parasitic while the other stages live free in the environment (e.g. in the vegetation, in manure, in the soil, etc.) and do not cause any harm to their hosts. Biting flies, fleas, mosquitoes cause damage to their hosts only as adults. Screwworms, flystrike, warble flies cause damage to their hosts only as larvae.

It is important to understand the life cycle of the parasites, because control measures consisting e.g. in applying insecticides on the host will affect more, less or not at all the stages in the environment that will continue attacking the host or producing infective stages.

Types of insects that parasitize livestock and pets

Parasitic insects of livestock, dogs and pets belong mainly to the following groups:

Ticks and mites are not insects but acari (or acarina), although they too belong to the arthropods.

Harm to livestock, horses, dogs and cats caused by parasitic insects

Many parasitic adult insects are blood sucking, i.e. they feed on the blood of their hosts, which is their main food and absolutely necessary for them to survive and/or to produce eggs. This is the case for stable flies, horn flies, mosquitoes, fleas, etc. Other parasitic adult insects do not suck blood (e.g. face flies, lice) but feed on fluids in humid parts of the body (eyes, nose, mouth, udders, genital organs) or feed on skin or feather debris.

Larvae of some parasitic insects cause the so-called myiases: the larvae feed on the host's tissues, either from outside (e.g. screwworm and blowfly larvae) or feed themselves into the host's body and migrate more or less inside while feeding (e.g. warble flies and human bot flies).

Harm to livestock or pets may come from painful biting and itching due to inflammation at the biting site; or to allergic reactions (especially in pets) that cause scratching. Intense scratching may produce loss of hair, wool or feathers as well as skin wounds that can become infected with bacteria or other microorganisms, or attract other parasites (typically screwworms and other flies). Blood loss due to heavy infestations may also cause anemia and negatively affect livestock performance.

Uncontrolled screwworms and blow fly larvae may literally devour their pray from the outside within days. Other myiasis caused by warble and bot flies may occasionally damage essential organs or just debilitate the hosts' organism.

Such harm often leads to substantial economic losses in livestock production. In the 1980's, the USDA estimated the annual losses in the US to the livestock industry due to biting flies to be about $500 million, those from lice about $40 million and those caused by other various insects about $300 milion. Economic loss due to ticks in Brazil was recently estimated to be more than $2 billion.

A very important harm caused by parasitic insects is disease transmission, since many parasitic insects are vectors of (i.e. they carry and transmit) microorganisms such as bacteria and viruses that are pathogenic for livestock or pets.

The most harmful insects for livestock and pets are listed below:

  • For cattle: particularly damaging insects with substantial economic impact are horn flies (worldwide, mainly range cattle), stable flies (worldwide, mainly for feedlots and dairy), tsetse flies (in Africa), screwworms (Central and South America, Asia), torsalo (Dermatobia hominis in Latin America), warble flies (Northern Hemisphere) and lice (worldwide). Houseflies may be a significant problem in the dairy industry worldwide.
  • For sheep: the most damaging insects with substantial economic impact are blowflies (mainly UK, Ireland, Australia, New Zealand and South Africa) and lice (worldwide).
  • For pigs: lice, stable flies and houseflies occur worldwide although damage is usually not as substantial as on cattle and sheep.
  • For poultry: lice and houseflies can have a significant impact in productivity.
  • For dogs and cats: the most harmful insects are fleas and mosquitoes that transmit various diseases.
  • For horses: the most frequent and harmful insects are bot flies as well as various flies and mosquitoes that transmit important diseases.

To visit an article on a single parasitic insect in this site use the corresponding menu or select one from the list below: