Animalia
(animals)
Fungi
(fungi)
Monera
(cyanobacteria, eubacteria)
Prostista
(protozoa, slime molds, algae)
Plantae
(plants)
4.1 Benthic Macroinvertebrate Taxonomy and Identification
Originally the term taxonomy referred to the science of classifying living organisms (now known as alpha taxonomy). Now the term is applied in a more general sense and may refer to a classification of things, as well as to the principles underlying such a classification.
In biology, the study of taxonomy is one of the most conventionally hierarchical kinds of knowledge, placing all living beings in a nested structure of divisions related to their probable evolutionary descent. Most evolutionary biologists assert an hierarchy extending from the level of the specimen (an individual living organism — say, a single fish), to the species of which it is a member (perhaps the Atlantic salmon), outward to further successive levels of genus, family, order, class, phylum, and kingdom.
Essential to this kind of reasoning is the proof that members of a division on one level are more closely related to one another than to members of a different division on the same level; they must also share ancestry in the level above. Thus, the system is hierarchical because it forbids the possibility of overlapping categories.
The level of classification goes from broad to more specific as you move down the hierachy. In general, each animal kingdom is broken into major groups known as phylum. Each phylum is divided into several classes, which are divided into several orders. Each order is divided into several families, which are then divided into several genera. Each genus can contain more than one species. Within species you have individual organisms. Over the years, as organisms are studied and discovered, there have been reclassifications and so it is common to see intermediate breaks between these levels to accommodate these changes. These breaks are denoted by prefxies such as sub-, infra-, micro-, parv-, or super- (e.g. subphylum, infraclass, microphylm, supervorder, parvorder).
Kingdom
Phylum
Class
Order
Family
Genus
Species
There are 5 kingdoms:
Animalia (animals) |
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Fungi (fungi) |
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Monera (cyanobacteria, eubacteria) |
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Prostista (protozoa, slime molds, algae) |
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Plantae (plants) |
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The benthic macroinvertebrates (BMIs) that we are apt to find in our waters are found under 4 main Phyla:
Note: taxonomic hierarchies can differ depending on the source and date of publishing. Here we have adopted the information as it was published on the Canadian Biodiversity Information Facility (CBIF)'s Integrated Taxonomic Information System (ITIS) pages (www.cbif.gc.ca).
Here is an example of the taxonomic classification of a common aquatic benthic macroinvertebrate species, Baetis tricaudatus:
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| Kingdom | Animalia |
Phylum |
Arthropoda |
| Subphylum | Hexapoda |
| Class | Insecta |
| Subclass | Pterygota |
| Infraclass | Paleoptera |
| Order | Ephemeroptera |
| Suborder | Pisciforma |
| Family | Baetidae |
| Genus | Baetis |
| Species | Baetis tricaudatus |
The life stages of BMIs found in freshwater systems are generally immature forms. Many live their adult lives on land, some only for a matter of hours. There are two main life cycle strategies of BMIs: complete and incomplete metamorphosis. These are important to understand with respect to the timing of BMI collection.
Complete metamorphosis (aka holometabolous) - 4 stages
egg - larvae - pupa - adult
The larvae do not resemble the adult form at all, and the pupae stage consists of the organisms inhabiting a "cocoon-like" structure where the main transformation from immature larvae to mature adult occurs.
This larval stage is the one which would be collected in freshwater systems, and some will not have all 6 legs visible. Instead, they will have pro-legs present and appear maggot-like.
Figure 4.1. Complete metamorphosis (www.kidfishbc.ca).
Incomplete metamorphosis (aka simple or hemimetabolous) - 3 stages
egg - nymph - adult
In this type of metamorphosis, the the immature form, the nymphs, do resemble the adult form (with the exception of wing development). The nymphs undergo sequential molting stages, called instars, until the final molt results in the adult (or imago in the case of mayflies). a series of aand the pupae stage consists of the organisms inhabiting a "cocoon-like" structure where the main transformation from immature larvae to mature adult occurs.
Figure 4.1. Incomplete metamorphosis (www.kidfishbc.ca).
What makes an insect and insect?
All insects belong to the phylum Arthropoda and are characterized by jointed legs, segmented bodies, and have a hard outer covering called an exoskeleton.
Identification of aquatic BMIs
There are 13 orders found in North America that contain one or more life stage of BMI that lives or is closely associated with water (Merritt et al.2008). Orders followed by an astreix (*) have almost all species having one or more aquatic stage.
Class and Order Collembola
Class Insecta
xxxxSubclass Pterygota
xxxx xxxxOrder Ephemeroptera (mayflies)*
xxxx xxxxOrder Odonata (dragonflies, damselflies)*
xxxxInfraclass Neoptera
xxxxxDivision Polyneoptera
xxxx xxxxOrder Orthoptera (grasshoppers and their allies)
xxxx xxxxOrder Plecoptera (stoneflies)*
xxxxxDivision Paraneoptera
xxxx xxxxOrder Hemiptera (true bugs)
xxxxxDivision Endopterygota
xxx xxxxOrder Neuroptera spongillaflies)
xxx xxxxOrder Megaloptera (alderflies, dobsonflies, fishflies)*
xxx xxxxOrder Trichoptera (caddisflies)*
xxx xxxxOrder Lepidoptera (butterflies, moths)
xxx xxxxOrder Coleoptera (beetles)
xxx xxxxOrder Diptera (flies)
xxx xxxxOrder Hymenoptera (wasps)
General morphology
Merritt et al. (2008) use a stonefly nymph to illustrate the general external morphology of an aquatic BMI since this is a more primitive group which is relatively unmodified and nonspecialized. Modifications are found within each aquatic BMI order - and all of these modifications will not be addressed here. A basic understanding of the overall general BMI body plan and terminilogy related to the external morphology is imperative for understanding and attempting any identification. Where possible, additional graphics will be provided to further illustrate and clarify various body parts and their modifications among the different BMI groups.
(modified from Holzenthal 2006)
Antennae
Antennae contain chemical, olfactory, tactile, and auditory receptors. Can also sense vibrations. There are many variations and modifications found across the aquatic BMI orders.
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Aristate antennae |
Capitate antennae |
Clavate antennae |
Filiform antennae |
Geniculate antennae |
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Pectinate antennae |
Plumose antennae |
Serrate antennae |
Setaceous antennae |
(Holzenthal 2006) |
Head
The head capsule of most immature aquatic insects is distinct, fully sclerotized, and visible externally as one of the three body regions (tagma). It bears the eyes, mouthparts, and antennae. In crane flies (Tipulidae), the head capsule is distinct, as it is in other nematoceran flies, but may be deeply incised ventrally or dorsolaterally and is partially or deeply withdrawn into the fleshy prothorax such that it is not visible externally except for the very apex.
In the more primitive brachyceran flies (e.g., Tabanidae, Athericidae, Rhagionidae, and Stratiomyidae among the aquatic flies), the head capsule is reduced posteriorly and only partially exposed externally; there are usually elongate internal metacephalic rods extending backward off the head.
In higher brachyceran fly larvae the head capsule is completely desclerotized. Only vestiges of the antennae and palps remain externally. Internally there is a highly specialized cephalopharyngeal skeleton retracted within the prothorax.
In larvae of parasitic Hymenoptera, the head capsule is weakly sclerotized and lightly pigmented. The mouthparts are greatly reduced and generally evident by characteristic cephalic sclerites; antennae and palps are reduced and often appear as small fleshy lobes. The mandibles, however, are present and functional.
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Diptera: head |
Hymenoptera: head |
(Holzenthal 2006)
Thorax
There are three distinct sections: prothorax, mesothorax, and metathorax
Legs
The thorax bears the paired jointed legs and wings. The forelegs are located on the prothorax, the midlegs and forewings on the mesothorax, and the hind legs and hind wings (if present) on the metathorax.
For the most part, larval and nymphal thoracic legs are composed of a maximum of 6 articulated segments:
1. coxa |
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2. trochanter |
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3. femur |
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4. tibia |
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5. tarsus (3-5 segments itself) |
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6. tarsal claw (1-2) |
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(Holzenthal 2006) |
Prolegs
Prolegs are defined as rounded fleshy usually paired tubercles on the prothorax alone, the prothorax and terminal abdominal segment, and/or on one, several, or all of the other abdominal segments. Prolegs are confined to the holometabolous larvae. Apically they have one or more spines or hooks. Prolegs are locomotory in function, but are not homologous to true segmental appendages.
Diptera have the greatest diversity of prolegs among the aquatic groups. In Trichoptera there is a terminal pair only, the anal prolegs, that bear a single terminal anal hook. Lepidoptera larvae typically have paired prolegs on abdominal segments 3-6 and 10 and these are tipped with crochets, small hook-like spines arranged in typical rows or rings. Corydalidae (Megaloptera) have a pair of anal prolegs only, but these are each tipped with a pair of anal hooks. Only a very few beetle larvae can be said to have prolegs, notably the ptilodactylid genus Anchytarsus, which has a pair of terminal multispined abdominal prolegs.
Some Diptera larvae (e.g., Tipulidae, Empididae) possess creeping welts, transverse, raised ridges on the anterior margins of abdominal segments 1-7 that bear tiny hooks. Blephariceridae and some Psychodidae larvae (Diptera) have suction discs which are used to hold the larvae in fast flowing water.
In addition to welts and suction discs, the abdomen of aquatic larvae may contain other fleshy protuberances, folds, filaments, or gills, but these are not locomotory and are not considered prolegs.
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Diptera: prolegs |
Trichoptera: anal prolegs |
Lepidotera: abdominal prolegs
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Megaloptera: prolegs |
(Holzenthal 2006)
Abdomen - generally composed of 11 abdominal segments. The dorsal part of the abdominal sections are called tergum, while the ventral sections are called sternum. These are joined laterally to the sides (pleura) by membranes.
Caudal filaments - the terminal section of the abdomen bears the anus at the apex, and the cerci to the sides. In the strict sense, cerci (singular, cercus) are paired elongate sensory appendages having their origin on abdominal segment 11 of the most primitive hexapods. Since segment 11 is highly reduced or absent in most orders, the cerci are more commonly articulated on segment 10.
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Some modifications to the caudal filaments include siphon and furcula.
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(Holzenthal 2006)
Lateral filaments
Larvae of Megaloptera have long tapering lateral gill filaments on abdominal segments 1-7 (Sialidae) or 1-8 (Corydalidae). This arrangement of filaments combined with a pair of anal prolegs each with 2 hooks in Corydalidae or with a single, medial terminal filament in Sialidae separate Megaloptera larvae from those Coleoptera larvae that may also have lateral filaments.
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Megaloptera: lateral filaments |
Coleoptera: lateral filaments |
(Holzenthal 2006)
Sclerite
A chitinous or calcareous plate, spicule, or similar part of an invertebrate, especially one of the hard outer plates forming part of the exoskeleton of an arthropod.
Mouth parts
Mandibulate or "chewing" mouthparts represent the generalized, primitive condition within the hexapods. They consist of 4 sets of appendages: the anterior labrum, followed by a pair of mandibles, then a pair of maxillae, and finally a posterior labium.
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(Holzenthal 2006)
This basic morphology evolved to form the more specialized mouthparts used for piercing and sucking, sponging, siphoning, etc (generally seen more clearly in the adult forms).
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Coleoptera: mouth |
Diptera: mouth with hook |
Hemiptera: mouth |
Megaloptera: mouth |
Odonata: mouth |
(Holzenthal 2006)
Gills
Gills are filamentous (single of highly branched) or plate-like outgrowths of the body which function to increase surface area for diffusion of dissolved oxygen across the cuticle and into a closed tracheal system. These occur in all aquatic orders except Hemiptera.
They commonly occur on the abdomen, but are also found on the thorax, especially in some stoneflies. Mayflies display a great diversity of tracheal gill morphology, including operculate gills that act as protective covers for more posterior gills. Caudal leaf-like tracheal gills are characteristic of damselflies, while internal rectal tracheal gills occur in dragonflies. In some beetle larvae (e.g., Elmidae) the filamentous caudal gills are retractile within an operculate chamber. The submental, cervical, and coxal gills of certain stoneflies probably function in osmoregulation rather than respiration, but for taxonomic purposes they are usually referred to as gills.
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Coleoptera: gills |
Ephemeroptera: gills |
Odonata: gills |
Trichoptera: abdominal gills |
Trichoptera: gills |
Trichoptera: thoracic gills |
Wing pads/wings
The number and size of wing pads, best developed in mature nymphs and pupae, correspond to the number and size of the adult's wings, e.g., Diptera pupae have a single pair of wing pads and the forewing pads of mature beetle pupae are thickend (the developing elytra).
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Trichoptera: wing pads |
Scutellem
The scutellum is the posterior portion of either the mesonotum or the metanotum of an insect thorax; however, it is used almost exclusively in the former context, as the metanotum is rather reduced in most insect groups.
In the Hemiptera, and some Coleoptera, the scutellum is a small triangular plate behind the pronotum and between the forewing bases. In Diptera and Hymenoptera the scutellum is nearly always distinct, but much smaller than (and immediately posterior to) the mesoscutum.
Elytra
The forewings of Coleoptera are highly modified into hardened, thickened, protective, shield-like structures, the elytra (singular, elytron).
The membranous hind wings are folded under the elytra in a manner unique to beetles. In most beetles, the elytra cover the entire meso- and metathorax and abdomen, but in a few families, notably the Staphylinidae, the elytra are short and the abdomen is exposed.
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Coleoptera: elytra |
4.1.4 Main distinguishing features among orders
(Holzenthal 2006)
Order: Ephemeroptera (mayflies)
Order: Odonata (dragonflies, damselflies)
Order: Orthoptera (grasshoppers and their allies)
Order: Plecoptera (stoneflies)
Order: Neuroptera (spongillaflies)
Order: Megaloptera (alderflies, dobsonflies, fishflies)
Order: Trichoptera (caddisflies)
Order: Lepidoptera (butterflies, moths )
Recommended Readings
Merritt RW, KW Cummins, and MB Berg. 2007. An Introduction to the Aquatic Insects of North America. Kendall Hunt Publishing. 862 pp.
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