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3. Introduction
Phototaxis is derived from the two words ‘Photo’ meaning light and ‘Taxis’ meaning the
movement of an organism in response to an external stimulus.
Therefore, Phototaxis can be simply defined as the locomotory movement of living organisms
directionally towards the source of light or away from the source of light.
Phototaxis is the innate response of an organism to variation in light intensity and direction.
Meaning that the more the intensity of light in a particular direction the more the phototactic
stimulation occurs in the organism.
4. There are various light-sensitive cells containing photopigments that mediate the Phototactic
response in an organism.
These light-sensitive cells are also called photoreceptors and the photopigments they include
are retinal (in rhodopsin), flavin (in cryptochrome), bilin (in phytochrome).
1. Positive Phototaxis is defined as the response of an organism in a direction towards the
source of light.
The source of light provides many organisms with both energy and information about their
surroundings, which is why these organisms commonly display locomotory movement towards
the direction of light, which is positive phototaxis.
Examples of phototrophic organisms exhibiting phototaxis are the various Phytoflaggellates,
Euglena, and photosynthetic bacteria.
5. 2. Negative Phototaxis is defined as the response of an organism in a direction away from the
source of light.
It has been noticed that Negative phototaxis can be only observed in larval Drosophila
melanogaster within the first three developmental instar stages. On the other hand, adult ones
can display Positive phototaxis very well.
6. Spodoptera litura: Oriental leafworm moth
● Also known as Cluster caterpillar, cotton leafworm, tobacco cutworm and tropical armyworm
● Undergoes holometabolous development with a life cycle of around 30-40 days
● Life cycle consists of following stages
● Egg Stage: The life cycle begins when a female Spodoptera moth lays eggs. These eggs are usually laid in clusters, often on the
underside of leaves or in other sheltered locations near potential host plants. but it typically lasts for about 2 to 10 days.
● Larval Stage: Once the eggs hatch, the larvae emerge. Spodoptera larvae are voracious feeders and can cause significant
damage to crops. They go through several instars (developmental stages) during this phase, molting and growing larger
between each instar. The number of instars can vary depending on species and environmental conditions, but it's usually around
5 to 6 instars. The larval stage can last from a few weeks to a couple of months, again depending on factors like temperature
and food availability.
● Pupal Stage: After completing their final larval instar, the larvae enter the pupal stage. At this point, they typically seek out a
sheltered location to pupate, such as in soil or plant debris. The larvae spin a silk cocoon around themselves, within which they
undergo metamorphosis into the adult moth. The duration of the pupal stage also varies but generally lasts around 1 to 2
weeks.
● Adult Stage: Once metamorphosis is complete, the adult Spodoptera moth emerges from the pupal case. These moths are
nocturnal and are often attracted to lights at night. Their primary purpose as adults is to mate and reproduce. Female moths will
seek out suitable locations to lay eggs, starting the cycle anew.
7.
8. Note: Larvae shoes positive phototaxis and move to the top of the plant which is more tender, light acts as a
positive factor for larval movement.
For pupation to occur, larva goes into the soil i.e. shows negative phototaxis. This occurs because pupa is a
non-feeding stage and hence goes into the soil
9. Procedure:
1. Cylindrical glass tube, opened from both the ends was taken and cleaned properly.
2. Using scale and marker, the tube was divided into 3 sections.
3. The tube is kept 5-6’ away from the light source
4. 10 larvae were first kept on wet filter paper for acclimatization and were then kept on dry filter paper
for the excess water to be removed.
5. These larvae were then carefully transferred in the middle of the glass tube.
6. Distribution of the larvae was carefully observed in each section i.e. stimulus section (nearest to the
light source), middle section and blank section (away from the light source).
7. Observation was noted down after every 2 mins
8. All the above steps were repeated for 3 replicates
12. Calculating %Orientational preference
% OP= (Mean no. of larvae at stimulus end−mean no. of larvae at blank end)/ Total no. of larvae moved
towards the tunnel
14. Precautions
1. Light source taken should be of medium intensity.
2. Size of the larvae should be approximately equal.
3. Recording of observations should be done carefully.
