The Medical Importance of Insects to Man

Insects are by far the largest group of organisms (Class Insecta) on earth. Whether measured in terms of numbers of species or numbers of individuals. Insects live in every conceivable habitat on land and fresh water, and a few have even invaded the sea. More than 70% of all named animal species are insects, and the actual proportion is doubtless much higher, because millions of additional forms await detection, classification and naming.

Most insects are relatively small, ranging in size from 0.1mm to head, thorax, and abdomen; three pairs of legs, all attached to the thorax; and one pair of antennae. In addition, they may have one or two pairs of wings. Most insects have compound eyes, and many have ocelli as well. The mouth parts of insects are elaborate. They usually consist of the jaws or mandibles, which are tough and unsegmented; a secondary pair of mouth parts, the maxillae, which are segmented; and the lower lip, or labium, which probably evolved from the fusion of another pair of maxilla-like structures. The upper lip, called the labrium, is of less certain origin. The hypopharynx is short, tongue-like organ (in chewing insects) that lies between the maxillae and above the labium, the salivary glands usually open on or near hypopharynx. Within this basic structural framework. the mouth parts vary widely among groups of insects, mainly in relation to their feeding habits.

Many orders of insects- such as Coleoptera, the beetles; Hymenoptera, grasshoppers, crickets, and their relatives- have chewing, or mandibulae mouth parts. In other orders, the mouth parts may be elongated or style-like. For example, in some flies (order Diptera) such as mosquitoes, black flies, and horse flies. there are six piercing, fused styles: the labrium, the mandibles, the maxillae, and the hypopharynx; the labium sheaths the stylets. In more advanced flies, the labium may be the principal piercing organ or may be expanded into large soft lobes through which liquid food is absorbed. The insect thorax consists of three segments (tagmata), each of which has a pair of legs. Occasionally, one or more of these pairs of legs is absent. Legs are completely absent in the larvae of certain groups- for example, in most members of the order Hymenoptera, the bees, wasps and ants- and among the flies, order Diptera. If two pairs of wings are present, they are attached to the middle and posterior segments of the thorax; if only one pair of wings is present, it is usually attached to the middle segment. The thorax is almost entirely filled with muscles that operate the legs and wings. The wings of insects arise as sack-like outgrowth of the body wall; in adult insects, they are solid, except for the veins. The internal features of insects resemble those of the other arthropods. Insects posses sophisticated means of sensing their environment, including sensory hairs to detect chemical signals called pheromones. Most young insects hatch from fertilized eggs laid outside their mother’s body. The zygote develops within the egg into young insect, which escapes by chewing its way out or by bursting the shell. During the course of their development into adults, young insects undergo ecdysis a number of times before they become adults and stop molting permanently. Most insects molt 4 to 8 times during the course of their development. The stages between the molts are defined as instars.

Metamorphosis
There are two principal kinds of metamorphosis in insects: simple and complete. In simple metamorphosis, the wings develop internally during the juvenile stages and appear externally only during the resting stages that immediately precedes the final molt. During this stage, the insect is called a pupa or chrysais, depending on the group to which it belongs. A pupa does not normally move around much, although the pupae of mosquitoes do move around freely. A very large amount of internal reorganization of the insect’s body takes place while it is a pupa or chrysalis. In the insects with simple metamorphosis, the immature stages are often called nymphs. They are usually quite similar to the Adults, differing mainly in their smaller size, less well-developed wings, and sometimes in their color. More than 90% of the insects, including the members of all of the largest and most successful orders, display complete metamorphosis, in which the juvenile stages and adults often live in distinct habitats, have different habits, and are usually extremely different in form. In these insects, development is indirect. Larvae in insects are immature stages, often worm-like, which differ greatly in appearance from the adults of the same species. Larvae do not have compound eyes. They may be legless or have legs as well as sometimes having leg like appendages on the abdomen. Pupae do not feed and are usually relatively inactive. As pupae, insects are extremely vulnerable to predators and parasites, they are often covered by a cocoon or some other protective structure. Groups of insects with complete metamorphosis include the moths, and butterflies; beetles; bees, wasp, and ants; flies and fleas.

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How to Photograph Insects and Other Tiny Creatures

Photography of Insects and Arachnids

You could hear her screams from three rooms away. We bolted to the kitchen to see what the problem was.

“What happened?”, we asked almost out of breath.

“It ran somewhere under there”, she panted back at us.

“What was it?” I questioned again

Nearly fainting, she managed to blurt out a stuttering, “I don’t know”.

Down on our knees now, we peeked into the space under the kitchen sink and cabinet.

It was a spider. Not even a big one at that. Although obviously big enough to get her adrenalin going. The slender, dark green body featured a series of yellow dots along its back.

“What kind of spider is that?” I queried. “We don’t know”, a few of the onlookers replied, “but those are common all over the place here”.

Insect Species World Wide

According to statistics there are in excess of two and a half million known insect species in the Amazon Rainforest region alone. Doubtless you’ve seen your fair share of the critters yourself. Insects in general are easy enough to identify.

Their general characteristics are:

- Have a body with three segments, a head, thorax and abdomen

- Have an exoskeleton

- Have two extended antennae

- are omnivorous, that is consume both plant and animal matter

Note Spiders are arachnids, and not technically insects. They have a body with two segments and eight legs, not six as do insects.

All this of course, meant nothing to “screaming Mimi” when she saw the spider.

Among numerous other characteristics, insects and spiders also make fascinating subjects for digital photographers. That is, if you’re not too skittish.

Getting good digital images of insects can be quite a challenge though. So here are some suggestions on how to successfully go about it.

Digital Photography Equipment Required to Photograph Insects

The two most important digital camera settings for this type of photography are macro imaging setting and your telephoto lens capabilities. Some cameras can also use close up lens attachments for shooting macro images up to one inch from the camera lens. A small but sturdy tripod less than one foot in total collapsed height is also on the suggested items list as are a flash and an external or slave flash unit if you have one available.

For Digital Photographers the Life Cycle of Insects

A quick word about the life cycle of insects for digital photographers is in order. Briefly, there are four key stages in an insects life cycle. These are helpful to know if you’re going to try your hand at getting digital images of insects like flies, mosquitoes, and an assortment of creepy crawlies from your yard or basement.

An adult insect lays fertilized eggs on its medium of choice; soil, plant stem or leaf, insect or animal carcass, dung, waste material, etc.
The eggs hatch into worms, caterpillars or larvae. The larvae of flies and other insects are called maggots.
The larvae feed voraciously for a period of time before they molt or change into young insects which resemble the adults. With butterflies, the larvae called caterpillars, weave a cocoon around themselves to complete a process of physical change, called metamorphosis, from the caterpillar stage into adult butterflies.
Young insects grow into adults, mate and lay eggs to repeat the life cycle

Due to the huge number of insect and arachnid species, there is a vast array of available subject matter regardless of where you might live. (OK, if you’re stationed in Antarctica, for example, you might be a bit limited) A great selection of insect macro images will make an excellent addition to your photographic portfolio in addition to expanding your ever-growing photographic skills. So set up a photo shoot with an insect or spider from your local area as soon as you can.

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Medical Arachnoentomology – Brief But Detailed Knowledge About the Insects

Insects
An organism having the general morphologic characteristics of an arthropod can be classified as an insect if the following criteria are met:
1. It has three distinct segments- head, thorax, and abdomen-with no evident effusion.
2. It has three pairs of legs (thus the term hexapods), each pair arising from one of three primitive thoracic segments (the prothorax in front, the mesothorax in the middle, and the metathorax posteriorly), which are ill defined in the higher species.
3. It has two pairs of wings, one arising from the mesothorax and one from the metathorax. Wings may be rudimentary or absent in some species.
4. It has one pair of antennae projecting anteriorly from the head segment.
5. Its mouth parts are designed for biting or sucking
6. Its abdomen is composed often similar-appearing the longitudinal segments, the terminal one of which is modified to include the reproductive apparatus and in some cases a stinger.

The sub-classification of insects, is based primarily on an assessment of the organism’s wings, mouth parts and type of metamorphosis.
1. Wings may be present or absent. There are species differences in the consistency, form, and size between the fore and hind wings. The venation (arrangement of the supporting ribs) of the wings is the most important identifying characteristic.
2. Mouth parts may be adapted for either biting or sucking. Species can be identified on the basis of structural differences in each individual mouth parts and their juxtaposition one to another.
3. In order to attain the size and development of the adult, the immature insect undergoes one or more changes in distinct stages.

Metamorphosis
Primitive metamorphosis: the least change, with the newly emerged young appearing similar to the adult except in size.
Simple (incomplete) metamorphosis: Three stages are recognized: the egg, the nymph, and the adult. During the nymph stage, the developing organism undergoes a series of molts, when wings or other appendages increase gradually to adult size. Complex (complete) metamorphosis: The newly hatched insect bears no resemblance to the adult, although careful examination reveals an internal anatomy that is distinctly insectile: The fly maggot is an example. An additional stage, the pupa, is required for the organism to accomplish the profound physiologic and structural changes between the larva and adult forms.

Although the medical technologist is rarely called upon to examine the internal structures of insects, it is of interest to know that they are highly developed despite their relatively small size. The nervous system consists of a chain of interlinked ventral nerve ganglia. The respiratory system is composed of a rather elaborate network of branching tracheal tubes which open directly to the outside as minute pores in the cuticle, permeating virtually all the internal cells with air. Detecting these structures may aid in differentiating insect larvae from small nematodes. With a hand lens or dissecting microscope. the openings of the tracheal tubes can be identified in an insect larva; the cuticle of the roundworm is continuous. The circulatory system is simple: a pulsating system, representing a primitive heart, is located anteriorly and attached to a large blood-filled cavity (the hematocele) by a single vessel (the aorta. There is no network of blood vessels or capillary channels. The digestive system is complete, with an oral opening anteriorly and an anal canal posteriorly. Pharynx, esophagus, stomach (or midgut), intestine, and rectum can all be identified under the microscope. In the bloodsuckers, the pharyngeal musculature is well developed so that it acts as a pump or siphon.

The genitals are also well developed; they include a pair of testes and seminal vesicle in the male; ovaries, oviduct, seminal receptacle, and ovipositor int he female. Four orders of insects of particula medical importance are: the siphonaptera (fleas), the Anoplura (lice), the Hemiptera (bugs), an the Diptera (flies and mosquitoes).

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