65 Lepidoptera – Introduction

Lepidoptera

The second mecopteroid order we cover in this lab is Lepidoptera (“lepido” = “scale”). This order is characterized by having the wings at least partially covered by scales. Larvae are mandibulate, but most adult forma have very reduced mouthparts. In most cases, only the galea of the maxilla remains, and it is modified to form a tube that acts as a siphon to feed on liquid nutrients. Some groups have the mouthparts completely reduced or absent, and do not feed at all as adults.

The majority of adult Lepidoptera have large wings that are coupled in some way to increase flight efficiency. In some groups, the front and hind wings are amplexiform, or overlap, to achieve synchrony. Other groups have a jugum, a lobe that projects from the rear margin of the forewing at the base, which interlocks with the hind wing. Most groups have a frenulum, a spine or group of stout hairs that projects from the front margin of the hind wing at the base. The frenulum connects with a row of hairs along the rear margin of the forewing. In a few species, females are brachypterous (“brachy” = “shortened”); examples include some species of Geometridae and Erebidae. Wing venation patterns are important for family-level identification in this order. You may also be glad to know that colour patterns have also been well-documented for most groups, and you will largely be relying on wing shape and colour to help you identify families in this lab.

There are about 150,000 species of Lepidoptera documented worldwide, with about 11,000 species known from North America. Some groups, especially the day-flying butterflies, are well-known, while many of the smaller, more cryptic and night-flying moths are less well-known – unless they are significant pests of agricultural or forestry crops. Larvae are mostly herbivorous, but at least one species from Hawaii, a Eupithecia sp., is a known predator of other insects as a larva, and another is a predator of snails. Adults, as previously noted, are mostly fluid-feeders, subsisting on nectar and/or minerals from mud, dung, and carrion. One lineage of the Noctuidae family contains known hematophagous members.

Lepidoptera have a similar system of sex determination as humans but, in the butterflies and moths, females are heterogametic (WZ), and males are homogametic (ZZ). Some species, in families, like the Psychidae, are commonly or entirely parthenogenic. Sexual reproductive behaviours are often complex, and commonly rely on chemical communication. The entire order is holometabolous, as are 80% of insects. Eggs are laid singly or in batches, sometimes on a host plant but often not. Many groups overwinter, in temperate climates, as eggs, with about 80% of the embryonic development already completed.

Lepidoptera larvae, or caterpillars, closely resemble larvae of the Hymenoptera suborder Symphyta, or sawflies. You should be able to tell Lepidoptera larvae apart from Hymenoptera larvae. Lepidoptera larvae have three pairs of jointed thoracic legs and, normally, five pairs of prolegs. Prolegs are extensions of the abdominal wall that act as support for the elongate body of the caterpillar. They are not jointed appendages. Each proleg in Lepidoptera has a row or circle of tiny hooks on the distal end, which the caterpillar can use to grip the substrate. Larvae of the family Geometridae have only two pairs of prolegs at the very end of the abdomen. When these larvae move they bring the back prolegs up to just behind the jointed thoracic legs, causing them to move like “inchworms” or “loopers”.

Larval Lepidoptera, despite their herbivorous habits, are often spectacularly well-defended from predators. Some sequester toxins from their food plants and use aposematic colouration to advertise their unpalatable or toxic nature. Predictably, there are also mimics of these unpalatable larvae. Other larvae mimic other distasteful or dangerous things such as bird droppings or even snakes. Some larvae also have urticating hairs or spines that can cause severe reactions in some people and domestic animals. A number of larvae produce repellent aromatic compounds on structures called osmeteria that can be extruded from between certain sclerites. Finally, the majority of Lepidoptera larvae are extremely cryptic, essentially the same colour as their host plants, and often exhibiting deceptive behaviours e.g. mimicking small twigs (Geometridae larvae are particularly adept at this!).

Most pupae are obtect, in the form of a chrysalis. In some groups, the last larval instar spins a protective cocoon before moulting to the pupal stage; in others, the pupa is buried in soil or attached to a leaf or stem of the host plant or another object.

The taxonomy and systematics of Lepidoptera are difficult, and many interpretations exist. In general, you will encounter two main subgroups, which are likely not monophyletic. The Monotrysia have the two genital openings combined; the only family you might notice around Prince George is the Hepialidae, or ghost moths. Hepialidae are also the only jugate moths you are likely to encounter. In the Ditrysia, the genital openings are separate; this group includes all of the families we will cover in the lab.

Lepidoptera Identification Aids

Identification of Lepidoptera adults is largely based on wing venation patterns, which can be difficult to see in insects that have their wings covered in scales! Identification to species often requires dissection of genitalia, a skill I am not going to attempt to teach. You may have noticed that no key is given in your text, either. Lepidoptera is, however, one of the few orders in which photographic comparison strategies work reasonably well for identification. Pay very close attention to details, though, and be sure to read descriptions, and look at distributions. Keep in mind that there are many families in this order that we do not cover in  the lab, especially among the micro-Lepidoptera.

The photos in your text, and the online sources provided here, should help you identify most of the specimens presented in the lab. You may need to extend your search in order to identify Lepidoptera families in your collections (especially small, brown moths!!). Below are some links to some sites that are helpful in narrowing down your identification. I am always happy to help you figure out what you have, too!

General:

E-Fauna: https://ibis.geog.ubc.ca/biodiversity/efauna/

iNaturalist: https://inaturalist.ca/

BugGuide: https://bugguide.net/node/view/15740

Lepidoptera-specific:

Moth Photographers’ Group: https://mothphotographersgroup.msstate.edu/

Pacific Northwest Moths: http://pnwmoths.biol.wwu.edu/

Lepidoptera of the Pacific Northwest: Caterpillars and Adults: https://www.fs.fed.us/foresthealth/technology/pdfs/FHTET_03_11.pdf 

I also include here a diagram of the standard Comstock-Needham system of wing vein nomenclature. If you wish to use a dichotomous key to identify any of the specimens in your collection, please let me know. There are a number of tips and tricks that I can share that will help you see various wing veins on a properly spread Lepidoptera without destroying the wings.

Bugboy52.40, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons