In this lab you will learn how immatures are different from adult insects and you will identify the immature insects in your collection using a KWIK-KEY.
The main objectives of this lab are to help you:
- learn to distinguish among nymphs, larvae, and adults
- identify (to order) the immature insects that you will submit with your insect collection.
You will need the following materials for this lab:
- Your collected insects
- KWIK-KEY to Immatures and Wingless Adult Insects
- Stereoscopic microscope
- ID labels
- Labeling pen
- Cork observation block
Immature insects come in three basic flavors:
- Young -- ametabolous immatures,
- Nymphs -- hemimetabolous immatures, and
- Larvae -- holometabolous immatures.
In Protura, Diplura, Collembola, Archeognatha, and Thysanura, the young are identical in appearance to the adults, but smaller in size. They are also sexually immature, but there is no way to tell this by looking at them.
Nymphs are usually recognizable as immatures because they have wing pads developing on the dorsal side of the thorax. These wing pads become larger and more obvious with each molt as the insect approaches maturity. Aquatic nymphs are often called naiads. Damselflies (Odonata), mayflies (Ephemeroptera), and stoneflies (Plecoptera) have external gills on the thorax or abdomen that make them easily recognizable as aquatic immatures. In hemimetabolous species with wingless adults (lice or bedbugs, for example), the nymphs may be rather hard to distinguish from adults.
Larvae are very different in appearance from adults. All of their physical features, including appendages, mouthparts, and internal organs, are assimilated and rebuilt into new adult structures during the pupal stage. Since larvae are so different from the adults, we usually give them distinct names such as maggot, white grub, wireworm, hellgrammite, caterpillar, etc. Larvae never have compound eyes: their primary visual organs are lateral ocelli (stemmata). You may want to review the five larval "types" you learned in Lesson #4: eruciform, campodeiform, elateriform, scarabaeiform, and vermiform.
There are four good "rules of thumb" that you can rely on for distinguishing adults from immatures:
- If an insect has functional wings, it must be an adult.
- If an insect has wing pads, it must be a nymph (or a naiad).
- If an insect has abdominal prolegs, it must be a larva.
- If a wingless insect has compound eyes, it cannot be a larva (it's either a nymph or a wingless adult).
1. Assemble your collected immatures, or insects you think might be immatures, whether they are in glass vials or pinned.
Your immature insects should already be preserved in alcohol according to the protocol you were given in lab #2. It may be necessary to remove insects from the glass vials and examine them under a microscope to see the characters needed for identification. Don't let the specimens dry out. Return them to the vial and replace any missing alcohol when you finish looking at them.
2. Identify your immature insects using the KWIK-KEY.
Use the "KWIK-KEY to Immatures and Wingless Adult Insects" as an identification guide. This key is similar to the dichotomous Order Key in Jacques and Bland that you used in Lab #2. The Kwik-Key, however, has more than two choices for some of the items. Start with question #1, choose the best answer from the options provided, and follow the arrow leading from that answer to the next question.
The KWIK-KEY works pretty well for most common nymphs and larvae, but it is not perfect. It may lead you astray on unusual or atypical specimens, and it is completely useless for pupae. Don't bother to include pupae in your collection unless they are distinctive (like mosquito pupae) or you have some other way of knowing what they are (like a blow fly pupa collected from carrion).
For help with identification, you can post pictures to the ID Discussion Forum or bring insects to my office. Either way, be sure to note where and when the specimen was collected -- that often helps narrow down the possibilities.
Your collection will be graded according to the checklist in the “Collection Instructions”. Please note the following points:
All immatures you submit for grading must be identified to order; you are NOT required to identify them to family.
If an immature has been pinned, be sure to include the word "immature" on its ID label. You will lose two points on your collection grade for each pinned immature that does not bear this label.
There will be no immatures for you to identify on the second lab practical, only adults.
Although identification of immatures to family is outside the scope of this course, you can still earn Family points for specimens that you identify and label correctly. Just include the family name under the order name on the ID label. Don't waste a lot of time doing this, but if you have a distinctive immature like an ant lion, a hellgrammite, or a hornworm, you can look up the family name and earn those points without much effort!
You will lose points on your collection grade for any of the following mistakes:
- Specimen is black and/or shriveled because it was not properly fixed in Kahle's solution. See “Preserving Insects in Alcohol”.
- Date/locality label missing or formatted incorrectly. See “Collection Instructions”.
- ID label missing or incorrect. See “Collection Instructions”.
- Failure to use "permanent" black ink for labels.
- Immature specimen is pinned without "immature" appearing on the ID label.
- Adult specimen is in alcohol without "adult" appearing on the ID label.