Insect any of the members of the class Insecta, the largest class of the phylum Arthropoda, which itself is the largest among the animal phyla. Of all the animal species so far described by science, insects account for about 85 percent; in addition to the nearly 10,000,000 insects species known, entomologists estimate that there might be more than the same number of species yet to be described.

Unlike most other arthropods, an insect has three body divisions: head, thorax and abdomen, covered by a hard exoskeleton, composed in part, of the protein chitin. The exoskeleton provides protection; it also serves as the point of attachment for the muscles. The exoskeleton is moulted periodically during growth.

Insects are unique because they have only three pairs of thoracic legs. Most insects bear two pairs of thoracic wings. Insects exhibit a bewildering variety of forms. They range in size from the barely visible but can be several cm in length or in wing spread. Their life spans range from a few hours to many years. They may be solitary or social. Insects feed on plants, animals and organic matters; their interactions with their food sources range from mutualism to parasitism to predation.

In many species females have an ovipositor, an abdominal appendage used for depositing eggs. Insects are instrumental in the pollination of many flowering plants (insects and flowering plants, in fact, evolved together). They promote the decay of organic material and the formation of soil; they form a major part of many food chains. Their interaction with humans is also various. Frequently, they are competitors for food either in the field or in storage. Some insects are carriers of plant, livestock or human diseases; some are nuisances; others infest and destroy buildings, furnitures and garments. On the other hand, insects produce such valuable commodities as silk, honey, wax and dyes; some are used as human food; some are valuable (parasites and predators) in controlling populations of other potentially harmful species; and some have proved useful in scientific and medical research.

Bangladesh is very rich in insect fauna having representatives of almost all the orders. The climatic conditions, mild winter, and bright sunshine are all favourable for insect growth and development. [Monawar Ahmad]

Insect parasitoid A specialized group of parasitic insects where the larva eats the living host, usually from within, eventually killing it. Parasitoids belong to several insect orders including Diptera, Homoptera, and Hymenoptera. Most parasitoids belong to Hymenoptera under the families Braconidae, Chalcididae, Ichneumonidae, Bethylidae, Pteromalidae, Encyrtidae, Eulophidae, Eupelmidae, Eurytomidae, Aphelinidae, Evanidae, Pompilidae, and Trichogrammatidae. Among the dipterans the families Muscidae, Tachinidae, Phoridae, Psychodidae, and Platystomatidae have parasitoid representatives.

In Bangladesh over 100 parasitoid species have been recorded from pests of different horticultural plants; agricultural crops, including rice, jute, and sugarcane; and from stored product insects. These parasitoids play an important role in suppressing the pest population in nature.

Insect parasitoids are minute in size, rarely measure more than 3 mm in length, and are variously coloured or black with metallic shades. In nature, most parasitoids are solitary but some are gregarious. Adults usually have two pairs of membranous wings with greatly reduced venation, the fore wings being larger than the hindwings. Some are wingless. Almost all hymenopteran parasitoids are characterized by geniculate antennae. Mouthparts are adapted for chewing or sucking. The ovipositor is usually well-developed and used in various ways for depositing eggs. Adults of many parasitoids feed on honey, glucose, rasins, nectar of flowers, etc. but some feed on body fluids of the hosts. In case of Pteromalid parasitoids, during egg-laying, the ovipositior forms a feeding tube. Some parasitoids deposit eggs without mating with the male. In such cases, the emerging adults are all males. The larvae of most species are parasitic on the eggs, larvae, pre-pupae, and pupae of other insects. Many species are mass cultured in laboratories and released for biological control of injurious insects.

Table  Select insect predators of insect and mite pests

[M Wahedul Islam]

Insect pollinator Insects associated with the transfer of pollen from anther to stigma, within a flower or between flowers. Insects visit flowers mainly to collect food i.e. nectar and or pollen. Besides bees, many other insects help pollinate crops. In Bangladesh about 70 species of pollinating bees have so far been reported. To pollinate Brassica species honeybees are being used to increase crop yields. However, it was observed that blowflies (order Diptera) were more suitable than honeybees for cross-pollinating selected lines of onion (Allium cepa). The Tiny Fig Wasp pollinates Ficus. The main natural pollinators of oil palm are three species of weevils of the genus Elaeidobius. Mango flowers are visited by many coleopterans, hemipeterans, dipterans, hymenopterans and thysanopterans. Wasps, moths, butterflies, thrips are also good pollinators. Pollination of banana is also done by the cockroach. Pollination by insects is called entomophily but among the different groups it varies, such as by beetles-cantharophily, flies-myophily, bees-melittophily, butterflies-psychophily, and moths-phalaenophily. [Md. Abdul Hannan]

Insect predator Free-living insects which feed upon other insects (prey) that are usually smaller and weaker, frequently devouring them completely and rapidly. Predators most often seek out and attack more than one prey for a full meal. Mantids, chrysopids, and many species of lady beetles are good examples of insect predators. Some successful cases of biological control in the world have been through insect predators. Generally, both immature predators and adults of many species are predaceous. [Md. Zinnatul Alam]

Leaf insect Common name for a foliage feeding, green, flattened insect of the family Phasmidae, order Orthoptera. The insect resembles closely the green leaf on which it lives in respect of its body shape and wing venation. All species have extremely flattened, irregularly shaped bodies, wings, and legs. Body length ranges from 4 to 8 cm. When at rest, the body almost perfectly matches the leaf of the host plant. Most leaf insects are tropical in distribution; Bangladesh has only a few species. They are not economically important. [SM Humayun Kabir]

Scale insect Any of the members of the superfamily Coccoidea, order Homoptera, that are generally considered closely related to the aphids. They are highly specialized group of insects of great economic importance. Their sucking of copious quantities of plant sap, by typically enormous, gregarious populations, causes the weakening and death of twigs, stems, leaves, fruits, and even the whole plant. The honeydew excretion of many of the Coccoidea, and the accompanying smut fungus, detracts from the esthetic appeal of woody ornamental plants.

Most species of scale insects are small, under 10 mm, and females generally lay eggs, but a few species give birth to live young ones. Newly hatched young scale insects are active and are often swept up from plants by the wind and blown far and wide, thereby spreading from plant to plant. The first instar nymph, often called a 'crawler' have legs, antennae, and usually search actively for an acceptable feeding site on the host plant. Females commonly lose their legs and antennae at the first molt, become attached permanently to the plant parts, suck sap from where they are localized, are rendered unable or uninclined to move, and secrete a waxy protecting covering, the scale. The adult male usually has a pair of wings, flies freely, and does not feed at this stage.

Table  Some scale insects and their hosts recorded in Bangladesh

Scale insects are of two types: (i) armored scales or Diaspidinae having distinct, hard, separable shells or scales over their delicate bodies, and (ii) soft scales, tortoise scales or Lecaniinae in which the hard shell is not separable from the body. Two species of scale insects are beneficial. One is the lac insect (Laccifer lacca) available in Bangladesh, which produces shellac, and the other is the cochineal insect (Dactypolius coccus) of Mexico, whose bright and red females are dried and powdered to make dye.

Scale insects comprise about 2,500 species, and are world wide in distribution. In Bangladesh over two dozens species have been recorded from different horticultural and agricultural crops. [Bidhan Chandra Das]