The following points highlight the two beneficial insects for us:- 1. Silk Moth 2. Lac Insect.
Beneficial Insect # 1. Silk Moth:
The silk moths are related to butterflies and belong to the order Lepidoptera. From the very ancient times, these insects are well-known for the ability to produce silk.
The rearing of silkworm for silk originally started in China, but now it is cultivated throughout the world. Generally, these moths lay their eggs on a particular tree, e.g. mulberry, and the caterpillars devour the leaves of that tree. But the most common silk moth, Bombyx mori, which is reared for silk, is absolutely a domesticated form and is very rarely found in wild state.
The adults are very short-lived and lay eggs on filter papers. The larva eats mulberry leaves and produce cocoon. Due to domestication, various parts of the body are degenerated. Another moth Antherareamylita is available in India, producing silk of little inferior quality. The application of biology which deals with the production of silk is called sericulture.
The moths are nocturnal and possesses a slender body which in silk moth is 2.5 cm. The body is divided into three regions —head, thorax and abdomen. The head contains a pair of compound eyes, a pair of bipectinate antennae.
In the butterfly, several appendages around the mouth form a proboscis which is coiled and funnel-shaped. But in silk moth the proboscis is absent and the first maxillae and labial palps are reduced. No mandible for cutting or piercing is present.
The thorax contains three pairs of walking legs and two pairs of wings. The wings are fairly large and cream coloured with brown lines on it. When at rest, it folds the wings over the body. The abdomen is well marked, segmented and contains no appendage. In females, it is slightly broader than males.
The mating takes place in shade and occurs immediately after the coming out of adult from cocoon. After mating the male die and female dies after the laying of eggs. In the wild state the female moths lay eggs on the leaves and branches of mulberry trees, but in domestication the females lay eggs on filter papers.
The eggs, popularly known as seeds, are minute and remain attached by means of a gelatinous covering produced by the female. The silk eggs art white in the beginning but in course of development becomes brown.
Within a short period, a small, pale yellow-coloured larva wriggles out of each egg. The larva, called a caterpillar, is a great eater and devours green mulberry leaves voraciously by cutting with sharp jaws. It grows rapidly and shades its external skeleton four times. This process is called moulting.
The caterpillar grows up to 7-5 cms and possesses a segmented body with three pairs of thoracic legs and five pairs of stumpy pro-legs on the ventral side. Ten pairs of spiracles are present for respiration. At the end of fourth moulting, a pair of silk glands develop on the lateral ride of the body cavity.
The caterpillar finally undergoes fasting and becomes fixed at one corner. At this stage it secretes silk from its silk glands and by the help of its mouth spins the silk around its body. The silk is sticky in the beginning but immediately after coming in contact with air stiffens and becomes fibrous.
Thus the body gets enclosed within a silken case called cocoon and is known as pupa. Within the cocoon the pupa transform’s rapidly, at first it becomes chrysalis and finally changes into a fully formed moth. When fully developed, the moth softens the one end of the cocoon and comes out of it.
Silk is obtained from the pupal cases which contain the pupa. First of all the pupa is killed by boiling and then the silk is unreeled from the case. Generally fibres are twisted together to form a thread. From a single pupal case nearly thousand yards of silken thread may be obtained.
Beneficial Insect # 2. Lac Insect:
The lac insects Tachardia belong to the order Hemiptera of the class insecta. Lac insects and its economic importance is known to mankind for a long time. The Atharba Veda contains an account and description of female lac insects. In Ain-E-Akbari specific account of the economic importance of lac insect is given.
Two species under the genus Tachardia namely T. lacca and T. chinensis are very common. T. lacca is the common Indian variety. India tops the lac producing countries and Thailand comes next to India. Lac insects live on certain specific trees like Kusum, Khair and Ber.
Nymphs developing directly from the fertilized eggs within the body of the female come out through an aperture called anal tubercular aperture. They come out in batches and the coming out from the older crust is known as swarming. About 150-400 nymphs come out from a single female. The nymphs are boat shaped and reddish in colour.
The body is divided into three regions—head, thorax and abdomen. The body of the nymph is short and measures 0.65-0.70 mm in length and 0.25-0.30 mm in breadth. Each nymph possesses three pairs of thoracic legs, one pair of antennae and a pair of caudal setae.
Some of the nymphs become winged or wingless males and others become wingless females. The mouth parts being piercing and sucking type the maxillae and mandibles become specialized. The males are short lived compared to the females.
The nymphs after coming out of the old crust roam about in search of soft branches of the tree in which they live. The nymphs settle in a suitable spot. It sucks the cell sap. It liberates a type of exudate. With repeated moulting the nymph gradually lose most of its body structures.
The thrown out skin together with the exudate form a crust around it. Each crust contains a pair of branchial pores for respiration and a big anal tubercular opening. In the male, the tubercular opening is provided with an operculum.
After three moultings the males come out by a removing the operculum and copulate with the female. Most of the males die soon after copulation. The female insect do not come out of its crust. The body of the female after being fertilized by the male assumes the shape of a hollow ball as most of the appendages are lost.
It starts laying eggs which remain buried under the body. The development is direct. From fertilized eggs emerge the nymphs which come out through the anal tubercle. The females die after the coming out of the nymphs.
Lac is a resinous exudate secreted from glands situated beneath the skin of lac insects. On chemical analysis it has been found that lac contains resin, carbohydrate, protein, water soluble salts, sand particles, volatile oils and pigments.
The twig containing mature incrustation is cut and washed thoroughly. The secreted materials from the twig is then scraped. The materials thus collected constitute granular lac. The granules are taken in suitable pot and the pot is heated by open charcoal fire. Dye is used at this stage to colour it suitably. The molten mass is stretched into sheet. After drying, these sheets are broken into pieces.
The commercial value of lac is great.
It is used for the following:
(a) Varnishes and polishes,
(b) Lithographic ink,
(c) Sealing wax,
(d) Insulating material,
(e) Shoe polish, toys and ornaments etc.