Saskatchewan Aquatic Beetles (Coleoptera)
The largest order of insects is the Coleoptera with over 250,000 species. Beetles are also one of the most diverse freshwater macroinvertebrate groups with over 5000 species, belonging to 18 families, having an aquatic stage in their lifecycle. Only nine aquatic families occur in Saskatchewan making up 230 species. About half of the species belong to the family Dytiscidae.
The beetles as adults are easily identified by the elytra or harden shell-like forewings. The size can range from 2 mm to 3 cm depending on family and species. The larvae are much more variable in form. Some have large fang-like mandibles while others have chewing mouthparts. The legs can be reduced or well developed. The abdomen can have lateral projections on it or dorsal ornamentation. Size can vary as much as the size as the adult.
The life cycle has four stages; egg, larva, pupa and adult. The number of stages that are aquatic varies among and within groups. However, with rare exceptions, the pupal stage is terrestrial.
Coleoptera are found in all types of aquatic habitats but the diversity is highest in lentic (standing) water. Evolutionary evidence suggests they invaded aquatic systems on many occasions. Each invasion resulted in different adaptations. This makes generalizations regarding ecology and life histories difficult. The Hydrophilidae have predatory larvae and the adults are omnivorous or herbivorous. Most Dytiscidae are predaceous in both stages. Predatory beetles either engulf their prey or grasp the prey with fang-like mandibles and inject digestive enzymes into the prey's body. The resulting "soup" is sucked out. Beetle larvae usually "breathe" through their integument (skin) or gills. Others use terminal spiracles that are protruded above the water surface. Adults carry an air bubble under the elytra or in hydrofuge body hairs that they must periodically return to the water surface to replenish.
Special mention should be made of the Dytiscidae. This family is the most diverse, largest (Dytiscus) and abundant aquatic beetle family in Saskatchewan. They inhabit all types of water bodies but due to their reliance on atmospheric oxygen are restricted to the shallow margins. Apparently, all species in the Saskatchewan have only one generation per year. Eggs are laid in spring by over wintering females and hatch soon after laying. The larval stage has three instars. Prior to pupation the final instar larvae crawl out of the water and dig a pupal chamber in litter along the shore. After a short pupation the adult emerges. Some of the larger species are able to live for more than one year in ideal conditions.
The dytiscids usually go unnoticed by the general public except in the fall and during the spring. In the spring many species disperse, as adults, from their overwintering habitats in rivers, lakes and larger ponds to the more temporary and smaller ponds that dot the landscape. These smaller habitats provide more food resources ie mosquito larvae and other insects and more heat units for faster growth. Breeding occurs and the larvae develop to maturity. By August the new generation of adults begin a migration back to their overwintering habitats. During this time there may be large numbers of beetles seen flying around, into and under lights and shiny surfaces. Unfortunately, vast quantities of these adults die due to this light attraction.
Another family of note is the Gyrinidae. This family is one of the most conspicuous and curious beetles found in Saskatchewan waters. Adults are usually seen in groups at the water surface swimming in a rapid and erratic manner. This behaviour gives them their common name of "whirligig beetle". The adult eyes are divided into upper and lower portions. This marvellous adaptation allows the beetles to live at the water surface and be able to see above and below without any distortion caused by the air/water interface.
Below is a family key to the larvae and adults of common aquatic beetles found in Saskatchewan. Please note that some uncommon larvae and adults are not keyed.
1a: Wings absent. Larvae.--2
1b: Elytra (hard fore wings) present. Adult beetles. Found in water or at times flying around lights.--Go to 10
2a: Legs made up of five segments.--3
2b: Larvae with only four segmented legs.--5
3b:Abdomen consisting of eight segments.--Dytiscidae
4a: Two claws at the end of each leg. Gill filaments present on abdominal segments. Four hooks at the end of the abdomen.--Gyrinidae (Note: Berosus (Hydrophilidae) has lateral filaments but lacks four claws at the end of the abdomen and has four leg segments.)-
4b: One claw at the end of the legs. Some have long spines on the top of the abdominal segments. Last abdominal segment formed into a long filament (A: Haliplus B: Peltodytes Note: spines on segments.) .--Haliplidae
5a: Antennae very long, much longer than head.--Scirtidae (Marsh Beetles)
5b: Antennae at most just slightly longer than head, usually much shorter.--6
6b: Abdomen with nine or ten abdominal segments. Abdomen usually with extensive hard plates.--8
7b: Head greatly reduced. Mouthparts not obvious. Legs greatly reduced. Maggot like.--Chrysomelidae (Leaf Beetles)
8a: Abdomen with nine segments.--9
8b: Abdomen with ten segments. The tenth segment is between two ceri.--Hydraenidae (Minute Moss Beetles)
9a: A ventral covering (operculum) (Inset shows operculum on ninth abdominal segment.) Found in flowing waters such as rivers and streams.--Elmidae
9b: No operculum on segment 9.~~~Some Hydrophilidae (Water Scavenging Beetles)
10a: Each eye divided in two. Usually shiny black elytra. Often seen swimming on the water surface in groups.--Gyrinidae
10b: Eyes normal, not divided in two.--11
11a: Plates covering almost the entire first leg segment and first few abdominal segments.--Haliplidae )
11b: No plates on ventral surface covering legs or abdominal segments.--12
12a: First segment of hind legs (coxae) dividing the first abdominal segment. Overall body shape streamlined.--Dytiscidae
13a: Maxillary palps long, visible dorsally. Antennae with last four segments in the form of a club (Arrow indicates club part of antenna starting with a "cup-like" segment.)--Hydrophilidae
15a: Antennae club comb-like. Segment two with an extension.--Dryopidae (Long-toed Water Beetles)
15b: Antennae thread-like. Last antennal segment may be slightly enlarged.--Elmidae
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White, D.S. and R.E. Roughley. 2008. Chapter 20. Coleoptera. In. Merritt R.W., K.W. Cummins and M.B. Berg. Ed. An Introduction to the Aquatic Insects of North America. 4th Edition. Kendall/Hunt Publishing Co. Dubuque, Iowa.