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Cockroaches are very hardy insects. It is known that, among other things, they are able to withstand bursts of ionizing radiation that would kill a human being.

The explanations of this observed resistance I've seen include cell division not being that fast in cockroaches, and the relative simplicity of these insects compared to other organisms. I know that microbes are able to resist radiation by having "tough" DNA (more G-C base pairs) and ready repair systems. Do cockroaches also have mechanisms like these, or is it really as simple as them being, well, "simple"?

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Most insects have similar resistance to cockroaches... IMO it is more that humans are weak than insects are strong. –  mbq Dec 15 '11 at 17:42
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@mbq Strong and weak are pretty subjective words. I could just as validly state my opinion that insects are biologically simpler creatures whose genome can withstand a larger amount of mutation and still be fully functional/viable arthropods, but I've no evidence to back it up. –  Lisa Dec 15 '11 at 23:19
    
@Lisa In relation to some median resistance not so much. Also note that this is a comment and I have made a disclaimer that this is my speculation. –  mbq Dec 16 '11 at 0:39
    
Some references on exactly how much radiation cockroaches can withstand in this article. –  horchler Jun 28 '13 at 20:50
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2 Answers

Off the top of head as a medical professional I can imagine the following mechanisms (everything is just speculative reasoning):

  1. Insects don't have blood. Instead, they have hemolymph whose primary role is not oxygen transport (they have an additional tracheal system for this purpose), but rather that of nutrients. Thus they don't need (and don't have) an intense proliferation of blood cell precusors -- these (bone marrow, spleen) are the most susceptible to radiation in a human and animal body.

  2. Insects have a rather primitive immune system that is mostly humoral[a] and much less cellular[b] compared to the immune system of animals and humans. This eliminates the next common weak place in the body: lymphatic nodes, thymus, again spleen and bone marrow etc.

  3. Insects have generally a much primitive and in many cases also rather decentralized nervous system: the ganglia are organized in a sort of a cord and even though the capital ganglia are usually larger, these dominance is not as prominent as in case of CNS and PNS in animals and humans. Therefore this system is much more tolerant to losses.

1.-3. Therefore, the only sensitive part of insects is the intestinal epithelium which gets renewed on a regular basis (similar to that of humans, also a known target of radiation), but...

  1. Insects (and generally the arthropodes) are known to have exoskeleton. This potentially serves as a good "armor" for vulnerable intestine cells, filtering out the most heavy particles (like alpha- and in some respect also the beta-particles). EDIT: This seems not to be real protection, see the discussion in comments.

Therefore it is not a surprise that insects generally show much higher resistance against radiation.

EDIT: As it was correctly added in the comments, there are also gamets, that are most sensitive to radiation (because they bear only the half of the normal genetic information and cannot repair mutations). Even though the lesions in gamets do not lead to immediate death, the potential sterility can easily cause the extinction.

However, cockroaches (and insects generally) are known to be r-animals, meaning that they favor the quantity (r) over quality (K) of their off-spring. This strategy is optimal when dealing with radiation-induced changes in gametes: the high number of offsprings compensates for the genetic imperfections in gametes.


[a] -- meaning that is has secreted peptides in their hemolymph that protect them
[b] -- there are phagocytes, somewhat similar to tissue magrophages in humans, but the rest of the cell chains in immune response in vertrebrates, like T- and B-cells, are completely missing. Those are responsible for the mediation and amplification of the immune response in vertebrates and are the cells that are most susceptible to radiation damage.

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It is a very good answer. However, I have reservations when it comes to the last point (i.e. exoskeleton). First, I would bet that thin exoskeleton screen radiation much less than human skin (the total amount is proportional to cross-section, i.e. ~mass/area (given the same atomic composition), which is greater for the skin). Second, alpha particles are likely to be stopped by both (a few cm of air suffices), beta needs a few cm of flesh (depending on its energy) so it is likely harm all volume of insect but not of human and gamma radiation won't be stopped by neither exoskeleton nor skin. –  Piotr Migdal Jan 23 '12 at 0:48
    
@PiotrMigdal You are probably right. What I was taking into account is that chitin can be likened to polysacharides that are known to have higher roentgenologic density compared to flesh. Can you confirm or disprove it? –  Alexander Galkin Jan 23 '12 at 12:38
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While it's not my main topic I bet that (as I said) for all typical organic substances the shielding is ~mass/area. And as chitin has density of ~1.5g/cm^3 (ref) 1 cm of flesh is equivalent to ~7mm of chitin. So for radiation which is neither stopped nor transmitted by both, insects (unless they are extremely large) are less shielded. However, there may be some nuances and I think it makes I good question for physics.stackexchange.com. –  Piotr Migdal Jan 23 '12 at 16:36
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Humoral and cellular immune system are parts of adaptive immune system, which is present only in vertebrates. Insect have only innate immune system that contain phagocytes and peptides (secreted by cells in fat body). You didn't mention about gametes, probably the most sensitive cells to radiation (becouse they are haploid). –  Marta Cz-C Jan 29 '12 at 22:46
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@MartaCz-C I use the words "humoral and cellular" in their direct meaning here, rather than referencing to the parts of adaptive immune system: this usage is acceptable and can be found in many research papers on insects (example). But thank for your suggestion, I will edit my answer. –  Alexander Galkin Jan 31 '12 at 10:50
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Studies of Deinococcus radiodurans, the most radioactively tolerant microorganism we know, show that it has many genes for DNA repair.

In the case of the cockroach, I would assume that in addition to repairing genes, and maybe some antioxidants produced in the cells to quench free radicals produced by radiation, the fact that the roaches lay many, many eggs is also useful. If the radiation kills 99.99% of the roaches, you will still have some insects coming from the egg clutches.

Selection is the insect's friend when you are a k-limited animal!

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