The Effect of Light on the Growth of Silkworms

Light mainly affects the tropism, behavior and lifestyle of silkworms, and then affects the growth and development of silkworms. Light is related to the life of silkworms mainly in three aspects, namely the quality of light, the intensity of light and the periodic changes of light. The quality of light refers to the different wavelengths of light, that is, color. The intensity of light is the illuminance of light. The cyclical change of light refers to the length of light in a day. Silkworms respond differently to three aspects of light at different developmental stages.

Silkworm’s response to light

When the embryo develops into the young silkworm queen in the egg, whether it hatches sooner or later and neatly depends on the light conditions. When using 3 different lightings of constant light, constant dark, and alternating light and dark, the latter hatches first and concentrates, followed by bright areas, and dark areas are scattered at the latest.

In addition to the factor of light time, the light intensity at the time of hatching also has an effect on hatching. If it is low light, it takes a longer time; if it is strong light, tens of seconds is enough. The hatched young silkworm is more lively in a bright environment than in a dark environment. It is more active under strong light (such as 400Lx) than weak light (such as <200Lx).

This phototaxis of young silkworms disappears completely after ingesting mulberry leaves for 10-20 minutes. If the silkworms go on hunger strike, the phototaxis will gradually recover. But the young silkworm fed artificial feed for 24h, the phototaxis did not disappear. Put the newly hatched silkworms in a petri dish full of the smell of fresh mulberry leaves, and the silkworms lose their phototaxis in a short time. The young silkworms that have lost their phototaxis are placed in a petri dish that does not smell of mulberry leaves. After about 3 hours, the phototaxis gradually recovers. All instar silkworms have a similar situation. The antennae of 5 years old were removed, and the mulberry leaves still showed lively phototaxis. It is speculated that the volatile attractant components in the mulberry leaves caused strong nerve excitement through antennae perception, thereby inhibiting the sensitivity to light and reducing the phototaxis.

Interestingly, this kind of inhibitory effect on silkworm phototaxis caused by mulberry leaves can be immediately relieved by electric shock. This result has been confirmed in all instar silkworms from 2 to 5 instars. It is known that electric shock can eliminate short-term memory in the central nervous system of animals including insects, which indicates that the inhibition of phototaxis is also involved in the central nervous system.

The phototaxis of each instar silkworm varies with the illuminance. For low illuminance below 100Lx, the phototaxis is positive, and for high illuminance above 100Lx, the majority is negative. In the same instar, the phototaxis of the silkworms is strong, the food and hypnotic silkworms are relatively weak, and the middle stage is the weakest.

Mature silkworm is the most active stage of phototaxis. It shows positive tropism for low illumination below 100Lx, and negative tropism above 100Lx. In the range of positive phototaxis, mature silkworms gather the most at an illuminance of about 13Lx, which may be the most suitable illuminance for it. The phototaxis of mature silkworms tends to vary from time to time. It is slightly weaker at first maturity, strongest at mature maturity, and then gradually weakened.

In addition to the difference in response to different illuminance, silkworms have different susceptibility to different wavelengths. The green light with a wavelength of 490 ~ 574nm is the most sensitive. young silkworms show positive phototaxis to near-ultraviolet light and yellow-green light, and have the largest response value to yellow light. Although the illuminance of white light increases, the response value of white light is very different from that of yellow light. Yellow light is mixed with red light, and the response value does not change at all. If yellow light is mixed with cyan light, the response value will quickly decrease as the proportion of cyan light increases. Mature silkworms have the strongest tropism to yellow light, followed by green, blue, and red light, and white light is the weakest.

Young silkworms have the strongest tropism for green light with a single wavelength of 557nm, which is related to the exploration or perception of green food-mulberry leaves. The green light passing through the mulberry leaves has the same remarkable attracting effect on young silkworms. Other experiments have shown that young silkworms have stronger tropism to mulberry leaves at the same distance under bright conditions than under dark conditions, while for young silkworms that lose their vision and phototaxis due to lack of vitamin A, no matter whether they are in bright or dark conditions, Conditions, the tendency towards mulberry leaves is very low. These facts show that in the series of actions of young silkworms that recognize food mulberry leaves, locate them, and reach the target, the phototaxis through the visual process is as important as the chemotaxis through the olfactory process. Means of action.

The phototaxis of silkworms is closely related to certain nutrients in the feed. When silkworms are raised with artificial feed (MO) that does not contain mulberry leaf powder, the silkworms gradually lose their phototaxis. When the silkworms are mature, almost all individuals do not respond to light. This is not that the overall mobility of the larvae is poor, but just that the receptivity of the eyes has deteriorated. This can be confirmed from the following identification results. The phototaxis threshold measured according to the light intensity phototaxis response curve of each instar and mature silkworm increases with development; electrophysiological measurement of monocular photosensitivity, 5th instar larvae raised with MO There is almost no electrophysiological response in a single eye.

The next generation of young silkworms produced by female moths raised on this feed also has no phototaxis. Adding mulberry leaf powder or adding B-carotene or vitamin A to this feed can restore the phototaxis of silkworms. It can be seen that this loss of phototaxis is caused by the lack of chromophores of the visual substance in the single eye, and the decline of visual function.

The effect of light on the growth and development of silkworms

Because silkworms have phototaxis, the light conditions in the silkworm house will affect the distribution of silkworms in the pedestal, which in turn affects the growth and development of silkworms. The silkworm house is dark, and the silkworms are scattered in the upper, middle and lower layers of the silkworm seat. The upper nibble leaves are fresh and have a higher temperature, which is easy to reach fullness, silkworm weight, rapid development, and dormancy rate; the lower nibble on withered leaves is easy to produce weak silkworms and late dormancy silkworms. Pay attention to the uniform light in the silkworm chamber, avoid direct sunlight or polarized light, so that the silkworms in the silkworm seat are evenly distributed, which has a good effect on the growth and development of silkworms.

Silkworm sleep has obvious daily rhythm. Silkworm rearing is based on the group of silkworms. Although the silkworms can go to sleep at any time within a day, it is best for the whole batch of silkworms to go to sleep in a short time during the day, so that the technical treatment is very convenient. Silkworms go to sleep in the morning and all will fall asleep during the day; for those who go to sleep in the evening, it takes 24 hours for them all to fall asleep. It can be seen that the sleeping and moulting activities of silkworms are suppressed from evening to late night, and promoted from morning to midday. The daily sleepiness is very strong for instar 1~2 , weaker for instar 3, and strong tendency to sleep from instar 4 to mature silkworm.

The results of feeding experiments with different photoperiods in the laboratory showed that the 24h photoperiod of light, short and dark can promote the growth and development of silkworms and shorten the age. The effect is most obvious in the test interval when the illumination is shortened from 21h to 6h. When the light is shortened to 3h, the instar is slightly longer.

Regardless of whether the large silkworm stage is naturally light-dark or dark, the whole cocoon and cocoon layers tend to be slightly heavier when the small silkworm stage is light-fed. The reason is that the long days of the small silkworm can make the silkworm develop in a uniform direction. After the whole instar is extended, the weight is heavier in the end, but the development is irregular.