My undergraduate student that I’m mentoring this semester asked me a question about when an insect is dead. He pondered this because we read a paper were researchers infected carpenter ants with a fungus were considered dead after no movement was detected. This got us thinking, if a brain is completely damaged or missing, is an insect alive? My thoughts immediately went to questions we’ve received here at Ask an Entomologist – specifically, the questions that asked, “why does a cockroach stay alive after its head is removed?” OR “why does a cockroach’s legs still move after its head is removed?”

For simplicity’s sake, the researchers needed a point of time to measure the death of the ant for their experiment. They knew that these ants were infected with a deadly fungus and would indeed, eventually die. In general, it’s easy to tell when an insect is dead because its legs are curled up and it is dried up.  However, I should backtrack because it’s not always “easy.”

Sometimes, it’s tricky because certain insects play dead. For example, this weevil pictured below is exhibiting an animal behavior called thanatosis (derived from Greek, “putting on death”). By pretending to be dead, it may be able to ward of predators. This is because many predators rather eat live prey and not something that’s been dead for hours to days (an animal that has been dead may have pathogens/diseases). Another example of thanatosis is in a hunting spider, Pisaura mirabilis. These spiders are ferocious predators. When wanting to mate with a female, the males must be careful to not seem like a prey item themselves. Like many spiders, male P. mirabilis will attempt to mate with a female after giving her a nuptial gift (i.e., an insect he caught and paralyzed). However, because of the nature of predators preferring live prey, he may be more appetizing than the nuptial gift. To avoid being mistaken as prey himself, the male hunting spider will enter thanatosis once he gives his gift to the female. If she likes and accepts it, he will pop back into action and mate with the female. Interestingly, not every male will feign death, but those that do are more successful at mating. Patience is a virtue?

Alight, but what about defining when an insect is actually dead? What about those cockroaches that are able to move without their head? A cockroach’s legs still move after its head is removed because of their thoracic (middle body) circuits are adequate enough generate leg movements; however, coordinated walking requires input from the head ganglia (Ridgel & Ritzmann 2005). Additionally, insects (e.g., cockroach) breathe through holes in the sides of their bodies called spiracles. Being able to breathe and move without a head means that a cockroach can keep kicking it until it eventually starves – which can, depending on species of roach, can take up to 50 days. 

It really comes down to semantics. As a definition, Merriam-Webster states that death is “a permanent cessation of all vital functions.” What are vital functions? I would consider that these are the ones that are essential for living. Insects are tricky to pin down (HA!) by this definition because many of their vital functions may continue functioning without their brain and/or head. This is due to the nature of their physiology. However, I would argue that although an insect may be able to move its body without its head, in all intents and purposes, it is dead because the head is essential overall. An insect needs its head to eat and because it releases hormones. However, when it comes to the nature of experiments this may need to be redefined because any synapses firing may indicate that an insect is alive. Experiments can be complex. For example, when measuring gene expression, it is important to consider as many spatial and temporal characteristics of the experimental set-up as possible. To play with semantics myself, surviving and living are different. Continuing to exist while hardly intact isn’t exactly living. Can a cockroach survive without its head? Yes. Can a cockroach live without its head? No.

Eisenstein, E. M., & Cohen, M. J. (1965). Learning in an isolated prothoracic insect ganglion. Animal Behaviour, 13(1), 104-108.

Pasteur, G. (1982). A classificatory review of mimicry systems. Annual Review of Ecology and Systematics, 13(1), 169-199.

Ridgel, A. L., & Ritzmann, R. E. (2005). Effects of neck and circumoesophageal connective lesions on posture and locomotion in the cockroach. Journal of Comparative Physiology A, 191(6), 559-573.

Rogers, S. M., & Simpson, S. J. (2014). Thanatosis. Current Biology, 24(21), R1031-R1033.

Willis, E. R., & Lewis, N. (1957). The Longevity of starved Cockroaches. Journal of Economic Entomology, 50(4).