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Order Lepidoptera - Butterflies and Moths

Phlyctaenia coronata -  Crowned Phlyctaenia 4953 - Anania tertialis 10495 Speckled Green Fruitworm Moth - Orthosia hibisci Tortricid moth - Olethreutes footiana Saddleback Caterpillar  - Acharia stimulea Notodontid? on gray birch - Nadata gibbosa Momphid, I think - Mompha solomoni - Pseudopostega cretea Peoria? - Peoria
Show images of: caterpillars · adults · both
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Lepidoptera (Butterflies and Moths)
Synonyms and other taxonomic changes
Explanation of Names
Lepidoptera means "scale wing", from Greek: lepis (λεπις / genitive form λεπιδος)- "scale" + ptera (πτερα)- "wing"
82 families and 13,044 species in North America (according to MPG), although there may be many unidentified species (Lepidoptera Bar Code of Life)
At least a couple of hundreds are introduced species. Bugguide lists 185 non-natives as of 10/25/2015
Adults have four membranous wings (rarely wingless); hindwings are usually smaller than forewings, both largely or entirely covered with scales. Adult mouthparts adapted for sucking, the proboscis is usually in the form of a coiled tube (adults of some species lack mouthparts and do not feed as adults). Images showing the characteristics of the order Lepidoptera:

Common practice is to divide the Lepidoptera into two (or three) groups, though this is not, strictly speaking, a taxonomic division. (Butterflies and skippers are monophyletic groups within the Lepidoptera, but "moths" are a paraphyletic group.)
Moths usually have feathery antennae and most are active at night. They generally rest with their wings open, either flat or "tented" over the body. When they pupate above ground they generally form a protective cocoon around the pupa. This is made of silk, often combined with other natural materials such as leaves or their own body hair. The caterpillars of many species dig into the ground to pupate.
Butterflies have thin antennae with "knobs" on the end and are generally active during the day. They rest with their wings closed above their bodies, and make a naked pupa also known as a chrysalis.
Skippers are a separate group of butterflies, with many distinctive features. They are (mostly) day-flying, have knobbed antennae, and rest with wings folded or spread, depending on the group.

Date/time-of-year and foodplant (for caterpillars) are sometimes helpful in determining the species.
Caterpillars are found in the habitat where their food is. Adults tend to be in the general area of their larval foodplants, but many migrate over great distances.
Caterpillars are active when their hostplants are plentiful, which is often spring and summer. Adults are usually seen when there are flowers to feed on. Because they over-winter, though, there are individuals of some species to be found any time of the year, even in cold climates. Seasonal patterns are usually species-specific, and some species can only be distinguished in the field by their season of activity.
Most adult moths and butterflies use their coiled mouthparts to suck nectar from flowers. In the process they may transfer pollen from one flower to another, and many plants depend on moths or butterflies for pollination.
Most caterpillars eat the leaves of plants (usually very specific kinds for each species). See World Database of Lepidopteran Hostplants.
Some carnivorous caterpillars have been documented. See: Carnivorous Lepidoptera
Life Cycle
These insects undergo complete metamorphosis; that is, each individual goes through four stages: egg, larva (the caterpillar), pupa (chrysalis or cocoon), and adult.
The larval stage does most of the eating and growing, with the adults often staying alive just long enough to mate and lay eggs.
Contrary to popular belief, butterflies and moths will not die if the scales are rubbed off their wings.
Please report significant date, location and/or species records to your local LepSoc Zone Coordinator
See Also
Trichoptera (caddisflies) tend to have hairs rather than scales on their wings; no coiled proboscis
Internet References
Moth Photographers Group - Main Menu (covers United States and Canada)
Butterflies of America - Intro (covers all of North and South America)
Butterflies and Moths of North America - Intro (covers United States only)
Butterflies and Moths of Canada (CBIF)
Lepidoptera Barcode of Life (covers the world)
Caterpillars of Eastern Forests (USGS) [dead link as of 20 Dec 2015]
Caterpillars of Massachusetts (photographs only)
Tree of Life (covers the world, but only as a taxonomic tree with a few sample images)