Post by Nancy Miorelli and Joe Ballenger

One day, while puttering around on Twitter we noticed that there was a #BugQuestions already…and there are some really neat questions here. The question of how insects breathe is one which is commonly asked, and it’s one which deserves some serious consideration!

“Do bugs breathe?” – @JoshPM #bugquestions

— Frank & Becky (@beckyandfrank) August 6, 2013

This is a naturally occurring transparent caterpillar! However, you can clearly see the the tracheal system connected to the spiraces.
Photo: Jim Cordoba, Enio Cano

Insects breathe oxygen, like us, but it’s generally not delivered to the tissues by the insect equivalent of blood. Instead, air is shuttled through a series of hollow tubes called ‘trachea’ which deliver air directly to their tissues. The trachea are connected to the outside by little holes in the insect’s exoskeleton called “spiracles”. The whole system is open, so the tubes that connect to the outside of the insect get smaller and smaller until, in some cases, they’re as small as a single cell. Oxygen just diffuses itself through the labyrinth of tubes into the insect’s system.

Check out the video below, by Nancy, to see how this whole process works.

This process is the same for nearly all insects, regardless if the bug ambles around on the ground or if it lives its life underwater. Mosquitoes get oxygen by breaking the surface tension of water (which is a lot harder than it sounds) with a specialized respiratory structure. They have to actively find a source of oxygen, and exploit it. Other insects make  scuba kits from bubbles.

Breathing Underwater
Left: A water scorpion uses extension of the tracheal system.
PC: Jeffdelonge, (CC BY-SA 3.0); Edited by Nancy Miorelli

Right: A diving beetle uses a bubble as a scuba kit.
PC: Wolfram Sondermann (CC BY-ND 2.0); Edited by Nancy Miorelli

In fact, here’s a really good video of a spider, called the diving bell spider, that completely encapsulates its abdomen in a bubble, and lives its life, hunting and spinning webs underwater. That’s right, you’re not safe from spiders anywhere.

I’m a Springtail and so small that I don’t need that fancy trachea equipment.
PC: Mvuijlst (CC BY-SA 3.0)

However, there are some insects  and life stages of insects which don’t have trachea. Insect eggs, for example, have small holes on their surface called aeropyles. These aeropyles act a lot like spiracles by allowing air into the developing insect. Insect eggs are generally small enough to breathe by simple diffusion, so a tracheal system isn’t necessary. Some insects are small enough to be able to breathe by simple diffusion, so they’ve lost a tracheal system. Although they’re not insects, springtails are probably the best known arthropod which have this system.

To the left, the Giant Peruvian Centipede eating a bat in its natural habitat. On the right, a Giant African Millipede on someone’s arm. Centipede picture courtesy of article cited below, millipede picture courtesy of Steve Jurvetson via Wikimedia commons.

This system places some serious limitations on their physiology, specifically on their size. The largest living animals with a tracheal system like this, the Giant African Millipede (Archispirestreptus gigas) and the Amazonian Giant Centipede (Scolopendra gigantea) don’t get much larger than a foot. In contrast the largest land-dwelling arthropod, the Robber Crab (Birgo latrus), uses modified gills to obtain oxygen and can reach 3 feet in length. Of course there are other limitations to insect size, but that’s a post for another time.

These are the challenges faced by 90% of insects, but there are other very complicated issues faced by that other 10%. Depending on how you slice the term, about 10% of insects have at least one stage of their life-cycle which is parasitic. Living inside other animals makes the simple act of getting oxygen a significant issue. Parasitoids are unique, because after they live inside their host as a larva, eat it from the inside out, they emerge from the dried husk of their former hotel to feed, mate, and then lay eggs in other unsuspecting living creatures.

This caterpillar has had better days…

So…how do parasitic insects obtain oxygen? Well, it has surprising parallels to the challenges faced by aquatic insects because they have similar problems. Like aquatic insects, parasitoids have to resort to creative methods to obtain oxygen in their unique environment. Think on it, and stay tuned for Part II!

References:

1. Klowden MJ. 2007. Physiological Systems of Insects (2nd Ed). Imprint, Academic Press. ISBN: 978-0-12-369493-5

2. Molinari J, Gutiérrez EE, Ascenção, AA, Nassar JM, Arends A, Márquez RJ. 2005. Predation by giant centipedes Scolopendra gigantean on three species of bats in a Venezuelan cave. Caribbean Journal of Science 41(2): 340-346.