Why do insects like cockroaches, spiders etc. fall on their back after they die?


Mostly insect mortality is caused by their exposure to chemical pesticides. Although several novel chemical pesticides have been developed and used in recent years, a majority of the chemical pesticides that we use today in agriculture and household pest management belong to organophosphates, carbamates and synthetic pyrethroids. These pesticide groups mostly kill the insects by adversely affecting the nervous system. For example, organophosphate and carbamate pesticides inhibit the enzyme that binds to the neurotransmitters.

Acetylcholine is a well-known neurotransmitter in the central- as well as peripheral nervous systems of several organisms including insects and human-beings. Acetylcholine is released from the vesicles in axon terminal and it moves by diffusion across the synaptic cleft to bind to the receptor on the other side. Acetylcholine is also located at the neuromuscular junction, controlling muscular contraction. Thus, acetylcholine sends messages between nerves, signaling muscle contractions. After sending the message, an enzyme called cholinesterase binds with acetylcholine, and allows the nerves and muscles to rest. If the enzyme cholinesterase is blocked, acetylcholine keeps on sending the same message repeatedly, and thus resulting in continuous over excitation of nerve to nerve and nerve to muscle communication. This might lead to hypertension, convulsions, paralysis and ultimate death.

Organophosphate and carbamate pesticides follow the above mode of action. That is, they inhibit cholinesterase and the resulting higher amounts of acetylcholine leads to the above symptoms in an insect. Similarly, synthetic pyrethroids also cause paralysis by keeping open the sodium channels in the neuronal membranes and thus leading to death of an insect.

When a normal insect falls on its back, it could correct its position with the coordinated movements of the legs and wings. However, it lacks partially or completely its ability to perform complex tasks through coordinated movements, when poisoned by the pesticides that mostly affect the nervous system. Because of the convulsions, the insect may often flip on its back, and move the legs in uncoordinated manner. It finally becomes paralytic before death, and hence it could not correct its position. If a normal insect dies due to injury, lack of nutrition or aging, it may also fall on its back because it lacks strength to correct its position