As some of you may or may not know, I am a contributing photographer at iStockphoto.com, a stock photography site. There approved contributing photographers upload inspected photos for sale to publishers, webmasters, or whoever needs them. To locate the photos they need, these buyers search the 3 million plus image library for a “Best Match” or other criterion based on keywords and phrases. These keywords, for the sake of clearer disambiguation, are mapped to preselected words or phrases. In other words, if I add the keyword “safe,” the system will ask if I mean safe as in safety, or safe as in a hardened box. The system works, though I was recently reminded that it isn’t perfect.
Nymph, Larva, Whatever
If you recall, I took photos of Dave Simser during his Grant Tick Hunt here in Nottingham NH. I decided to queue up a couple of these for my iStock portfolio. No problem… that is until I disambiguated the keywords I was adding. One of the photos was of a macro shot of a deer tick nymph, so naturally the keyword “nymph” was applied. That’s when things went bad. The word nymph was mapped to three choices: “Nymph (Classical Mythology Character),” “Elf (Speculative Being),” and “Larva (Animal Stage).” Well, that is all fine and good, Larva (Animal Stage), is the best match, but a nymph is not really a larva. I am being pedantic; to some a bug is a bug, but not everyone sees it this way. To some the distinction is important, and a “bug” is not always a bug.
It’s no big deal, the search and disambiguation system over there was built by humans and is being constantly improved. And humans are fallible. It did make me think, though. If there is confusion over there about the distinction between a larva and a nymph — something I thought was common knowledge — maybe there’s confusion elsewhere, too. Thus I decided to write this — to provide a couple of entomology basics, touching on life cycles and terminology used to describe them.
Metamorphosis
Arthropods such as insects and mites develop in two basic ways, with some variations. This development is called metamorphosis, and it comes in two versions: complete and incomplete.
Complete Metamorphosis
This is the development type most people are familiar with as it is the one we are taught in grade school. Most typically the butterfly is given as an example. A butterfly mates and lays some eggs, each one will hatch into a caterpillar (larva, larvae is plural), will undergo a transformation as a chrysalis (pupa, pupae is plural), then emerge as an adult to repeat the cycle of life. Many follow this route as well. Bees and wasps, flies, beetles, and others.
The four stages are quite distinct and different. The egg stage is just as you probably envision it, the larval stage is usually quite featureless compared to the adult form, the pupal stage is quite different from the larval stage as it happens within a protective enclosure of some sort. Cocoon, honeycomb cell, etc. It is in this stage that a huge change occurs. To emphasize the difference, look at how different a butterfly is to a caterpillar.
Most of the growth development is carried out in the larval stage (that’s why caterpillars eat so much). As they progress through this stage they go through sub-stages which are known as instars. Instars are usually identified by a numeric designation. For example, first instar, second instar, third instar, etc.
Incomplete Metamorphosis
This development type is less well know, hence the “nymph” disambiguation issue I mentioned previously. Incomplete metamorphosis is quite different from its counterpart. I mentioned that a bug is not a bug to some. This is because bugs are actually a specific type of insect. It is referred to as a “true bug.” They usually have a straw-like mouthpart or proboscis, An example would be a soldier bug or our Orius insidiosus. Bugs, as they are now known, undergo this metamorphosis type. Mites, which aren’t even insects, also undergo an incomplete metamorphosis. Aphids, which are not a “bug,” are another. You can also include mantids, grasshoppers, and many others. And, of course, ticks.
In incomplete metamorphosis the adult will usually lay an egg, but even at this point some of the similarity ends because some insects will have a live birth (the egg hatches within the mother), aphids being a prime example. In either case, the egg develops into a small, less distinct yet recognizable version of the adult. A nymph. This is different from a larva because the larva will look different from the adult form. In order for a larva to look like an adult is relies on pupation. The nymph has no need of pupation. An incomplete metamorphosis skips the pupal stage altogether.
The nymph progresses along much as a larva does with its instars, progressing past various milestone stages, moulting several times as it grows. The final step leads to full adulthood.
Set in Stone?
There will always be some blurring of the lines, variations, and it can be confusing if one looks at it too deeply, especially as pertains to terminology, but hopefully one can now know the basics.
Lucy Corrander responds:
Posted: July 31st, 2008 at 12:20 pm →
Hello
I found you through Blotanical and I’ve been bouncing around, looking at the photos rather than reading the text - but I’ll do more of that another time.
I’ve enjoyed being here.
Lucy
Mike Cherim responds:
Posted: July 31st, 2008 at 12:38 pm →
Hi Lucy. Glad you like the site. I did remove the redundant link to your site (nice photos by the way) because your name links to it already.
LetsPlant responds:
Posted: July 31st, 2008 at 7:13 pm →
I found your blog on Blotanical. It is fantastic! I am going to add it to my blogroll so I remember to check back often.