Lab 1:
Arthropod Collection & Identification
PRE-LAB CONCEPTS
Introduction
In this activity, you will embark on an expedition to collect arthropods from local fauna, the animals present in a particular region, habitat, or time. Prior to the activity, review the scientific method and develop a hypothesis with a sampling scheme for determining which organisms (individual life forms such as plants, animals, and bacteria) may or may not harbor the bacterial endosymbiont, Wolbachia. An endosymbiont is an organism that lives within the body or cells of another organism; Wolbachia is one of the most successful endosymbionts on the planet and infects up to 60% of all arthropods. Sampling locations should be coordinated with your group and could include your home, nearby nature park, schoolyard, or carefully selected habitats that differ in temperature, location, etc.
A testable hypothesis can be proved or disproved based on scientific research. If you are performing a biodiversity or discovery-based experiment, first think about why, when, and where you are sampling. What do you expect to find? Do you expect all arthropods to be infected with Wolbachia? Develop a testable hypothesis prior to your experiment. For example:
- XX% of the arthropods on my school campus are infected with Wolbachia.
- Arthropods collected from leaf litter are more likely to be infected than freshwater arthropods.
- There is no difference between the Wolbachia infection prevalence of Coccinellidae (ladybird beetles) in the spring and fall seasons.
- Ants nesting in the soil are Wolbachia-positive, but tree-dwelling ants are not.
After arthropods are preserved in ethanol or rubbing alcohol, you will identify each specimen to taxonomic order and create an entry in The Wolbachia Project Database. This will enable further exploration of new questions, such as:
- Why do species in some insect groups have few or no endosymbionts while insect species in other groups have these microorganisms in every species examined to date?
- Suppose you find that a collection of ants nesting in soil have Wolbachia, whereas the ants that live in the trees above do not. Does this indicate that soils in some way contribute to how Wolbachia transmits itself among species of ants? Can this hypothesis be supported if both soil-dwelling and tree-dwelling ants have Wolbachia?
Without knowing the identity of the arthropod host these ideas may never be explored, and certainly could not be tested. Therefore, it is essential to (i) upload a high-quality picture for each arthropod that will be screened for Wolbachia, (ii) provide clear and concise observation notes in the database, and, if possible, (iii) include DNA sequence data.
Recommended Resources
iNaturalist is a crowdsourced species identification app that is used to record biodiversity observations and help naturalists identify organisms in their local environment. The software will suggest options for identifying your arthropod and, after submission, citizen scientists will provide feedback on the classification.
OpenStax is a free online textbook. Chapter 28.6 of the Biology 2e textbook provides an overview of the phylum Arthropoda, including descriptions for each of the subphyla discussed in this lab activity.
Taxonomy
Taxonomy is the classification of organisms based on relatedness. Developed by Swedish botanist Carl Linnaeus in the 1700’s, scientists across the world use taxonomy to organize all living things in a hierarchical structure. Because there are over one million named species of arthropods, it can be difficult to classify beyond order based solely on visual observation; therefore, DNA sequence similarity is often used alongside morphology (form and structure) and ecology (the study of the relationships between organisms and their physical environment) to determine classification. The scientific name for an organism is based on binomial nomenclature, a unique two-word name combining the genus and species. It is written in italics and the genus is always capitalized. This name can be shortened by only including the first initial of the genus, as shown in the table below.
* The above classification is based on the three-domain system of taxonomy (Archaea, Bacteria, Eukarya). However, increasing evidence supports a two-domain system. First proposed as the eocyte hypothesis by James A. Lake and colleagues in 1984, Eukarya are included with the Archaea.
Arthropods
Arthropods, the most diverse and abundant animals on earth, are characterized as invertebrates with a hard covering called an exoskeleton, segmented body, and jointed appendages. They represent about 85% of all described animal species and there are an estimated 10 quintillion (10,000,000,000,000,000,000) individual arthropods inhabiting the planet on any given day. If the services these animals provide everyday (for free) suddenly disappeared, humans would soon disappear. They clean water, pollinate flowers that produce about 1/3 of the food we eat, break down waste, and decompose plants and animals. Given their importance, why are they so little appreciated and poorly understood? Perhaps the single largest factor is their small size. Most of the one million described species are less than 1/3” long. This entire world of small creatures exists literally under your feet.
The phylum Arthropoda is classified into five subphyla (plural of subphylum, a taxonomic category that ranks below phylum and above class):
- Chelicerata: spiders, scorpions, mites, ticks, horseshoe crabs, and relatives
- Crustacea: crabs, lobsters, crayfish, shrimps, krill, barnacles, prawns, woodlice, and relatives
- Hexapoda: insects, springtails, and relatives
- Myriapoda: millipedes, centipedes, and relatives
- Trilobita: trilobites (extinct)
What is the difference between an arthropod, spider, insect, or bug? Taxonomy. True bugs belong to the order Hemiptera and include bedbugs, assassin bugs, and stinkbugs. All bugs belong to the class Insecta, which are characterized by three body segments (head, thorax, abdomen) and six legs. Spiders, on the other hand, belong to the class Arachnida and have two body parts (cephalothorax and abdomen) and eight legs. All insects and arachnids belong to the phylum Arthropoda.
Insects
Most arthropods are insects, members of class Insecta and subphylum Hexapoda. Hexa and poda both originate from Latin (from Greek) meaning six and those having feet, respectively. All insects have three body segments.
- The head features conspicuous eyes, mouthparts, and antennae.
- The thorax contains wings and legs.
- The abdomen contains the reproductive organs, where many of the endosymbiotic bacteria reside.
Of course, it is common for there to be exceptions to the body plan. Many insects lack wings or legs (particularly in the larval stage), some are eyeless, etc. These features allow for classification into groups. Order is the most familiar taxonomic grouping.
Common examples are:
- Order: Blattodea – cockroaches and termites
- Order: Coleoptera – beetles
- Order: Diptera – flies
- Order: Hemiptera – true bugs
- Order: Hymenoptera – bees, wasps and ants
- Order: Lepidoptera – moths and butterflies
- Order: Mantodea – praying mantids
- Order: Odonata – dragonflies and damselflies
- Order: Orthoptera – grasshoppers and crickets
- Order: Phasmatodea – stick insects and leaf insects
- Order: Phthiraptera – biting and sucking lice
- Order: Siphonaptera – fleas
Notice that most, but not all, insect orders end with the suffix “-ptera.” Ptera is the plural form of the Ancient Greek pterón, which means wing or feather. Wings are one of the most conspicuous ways orders of insects are differentiated.
For the purpose of this lab, it is recommended to study smaller species because they are easier to dissect and preserve better. Larger insects rot more quickly, thus degrading DNA and making it more difficult to examine bacterial endosymbionts.
LAB GUIDES
ADDITIONAL RESOURCES
FAQs
