Flat mayflies!

There are all kinds of flat insects in fast flowing streams.  In the White Mountains of Arizona, you can find a few types of flat mayflies alongside the water pennies on the same submerged rocks.  Take a look at these photos:

heptageniid

Flat headed mayfly, bottom view

heptageniid

Flat headed mayfly, bottom view

This mayfly belongs to the flat headed mayfly group and is REALLY flat!  Bug legs tend to curl up when they’re preserved, like in this specimen, so this mayfly would actually be much flatter than it appears here if it were alive.   Flat headed mayflies have several adaptations to flow that you can see in the images above.  First, take a look at the gills, the plate-like structures sticking off the sides of the back half of the bug.  They stick out from the side of the body rather than up like they do in many mayflies.  This helps them keep their gills close to the surface of the rock and inside the boundary layer .  Second, when these insects are alive, they keep their legs held far away from their bodies and absolutely flat against the rock.  These bugs have enormously long legs, but they are also very flat, so they are able to fit them within the boundary layer too.  Finally, they have big, broad, flat heads.  They keep these pushed against the rock, within the boundary layer as well.  The whole bug is only a few millimeters thick, even though they can be close to an inch long!  These are probably some of the flattest bugs there are.  It is a great adaptation to living in a high flow aquatic habitat.

Flat headed mayflies move in a strange way.  Unlike the water pennies, which keep their legs tucked under their bodies and walk along the rock much like other insects do, flat headed mayflies hold their legs flat against the rock and far away from their body.   This makes it hard to walk.  In fact, they tend to shuffle along the rock rather than walking.  Imagine wandering across the floor on all fours.  This is how most insects walk, with their bodies held far away from the surface they’re walking on.  It’s quick and efficient.  Now imagine lying flat on your belly with your legs behind you and your arms out to your side, then crawling commando-style with your body only an inch above the ground.  It’s a lot harder to do, right?  Flat headed mayflies don’t move very quickly or very gracefully.  However, if they pick their bodies up off the rock, they risk getting caught in the current and being swept downstream.  So, they keep their legs close to the rock and push themselves across the rock by pushing with the legs in the opposite side of the body from the direction they wish to go.  It’s not the most efficient way to get around, but it works for them because it helps keep them safely within the boundary layer of their rock.  There probably aren’t many predators that are going to pick them off of rocks in very fast flowing water either, so moving quickly is not as big of an issue as it is for many other insects.

Other aquatic insects have different adaptations to flowing water.  I’ll discuss some of them in future posts.  Next time, however, I’ll talk about why I call all insects bugs and what a bug really is.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

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Glossary

Abdomen: the back section of the insects, behind the point of attachment for the wings and legs.

Ametabolous: in insects, a type of metamorphosis in which all stages but the egg look the same apart from size

Boundary layer: the layer of low flow that occurs when fluids (water, air, etc) flow over the surface of objects in the stream.  See the Water Penny entry for a more detailed description of how a boundary layer works.

Bug: an insect that belongs to the order Hemiptera.  See the post about bugs for more information.

Complete metamorphosis: see holometabolous

Decomposer: an organism that helps break down a dead organism

Elytra: the hardened forewings of beetles.  Elytra means “sheath” in Greek.  See the post about bugs for more information.

Entomologist: a scientist who studies insects

Exoskeleton: the hard, tough outer covering of insects

Exothermic: organisms that are largely unable to produce their own body heat and regulate their temperature mostly through behavioral means

Flier: in odonates, the behavioral group which includes insects that rarely land and observe their territories on the wing.  Contrast with perchers.

Forewings: the front pair of wings in insects with wings.

Hemielytra: the specialized forewings of the true bugs (Order: Hemiptera) that are leathery on the upper half and membranous on the lower half.  Hemielytra means “half sheath” in Greek.  See the post about bugs for more information.

Hemimetabolous: insects that undergo incomplete metamorphosis and have a life cycle that consists of egg, nymph, and adult

Hemiptera: the order name for the group containing the true bugs.  All members of this group have hemielytra and piercing-sucking mouthparts.

Hindwings: the back pair of wings in insects with wings

Holometabolous: insects that undergo complete metamorphosis and have a life cycle that consists of egg, larva, pupa, and adult

Incomplete metatmorphosis: see hemimetabolous

Instar: the stage between molts in immature insects

Immature: an organism that is not fully developed and not reproductively capable

Insect: an animal that has 3 body segments (head, thorax, abdomen), six legs, two pairs of wings (may be missing in some insects), and one pair of antennae

Larva: the immature stage of holometabolous insects

Metamorphosis: in insects, a process by which an insect changes from one stage and/or form to another.  See also holometabolous, hemimetabolous, ametabolous, and the entry on metamorphosis in insects.

Molt: in insects, breaking out of an exoskeleton that has grown too small and expanding the new, larger exoskeleton beneath; the stages between molts are called instars

Nymph: the immature stage of hemimetabolous insects

Odonata: the order name for the group containing the dragonflies and damselflies.

Odonate: an insect belonging to the order Odonata

Parasite: an organism that lives off another organism to the detriment of the host organism and the benefit of the parasite

Percher: in odonates, the behavioral group where the insects sit on perches and observe their territories while resting rather than on the wing.  Contrast with flier.

Piercing-sucking mouthparts: the specialized mouthparts of true bugs that inject digestive chemicals into their food, then suck the food up into their mouths after the chemicals have liquefied it

Predator: an animal that eats another animal

Prey: an animal that is eaten by another animal

Pruinose: in dragonflies, individuals with a waxy and/or dusty looking coating on their bodies

Pupa: in insects, the developmental stage between larva and adult where a complete rearrangement of tissues occurs

Sclerotization: in insects, a process by which a soft, flexible exoskeleton is hardened into a hard and/or tough covering

Spore: the reproductive body of molds that allows molds to spread to new areas

Territory: an area containing necessary resources (food, access to mates, habitat, etc) that an animal is able to protect sufficiently to exclude other individuals at will

Thorax: the middle segment of insects, where the wings and legs are attached

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Text copyright © 2009 DragonflyWoman.wordpress.com