I’ve been noticing lately, and maybe you have been too, what seems like an awful lot of insect frass around. Frass on my porch, my roof, my adirondack chairs. Put up a tent or a tarp, and as long as it is underneath some trees, it will be covered with the stuff. What this tells me is that there are a lot of insects in the trees, specifically, larval insects, and they are apparently doing a lot of eating. Frass is indeed linked to food, because frass, my friends, is insect poo. Just like you and me, the more they eat, the more they excrete.
Summer is the time of the insect, the warm temperatures bring plentiful food sources, and insects have evolved to take full advantage of this brief window. The insect life cycle is a fascinating one; I, for one, can’t stop thinking about how cool it is. There are three main paths to becoming an adult insect. The first is simple metamorphism, and it has a couple of sub categories. Of those, the first is gradual metamorphism (also called paurometabolous (small or slight change) metamorphism). The egg is laid by an adult, and what hatches out of the egg is essentially a tiny little adult, that eats and grows and molts its exoskeleton and grows some more, molting and growing until it gets big enough to be sexually reproductive and is considered and adult. This is the M.O. of grass hoppers, for example. The other form of simple metamorphosis is incomplete or hemimetabolous (half change) metamorphism. An egg is laid by an adult insect, and out of the egg hatches a juvenile stage that looks different than the adult. In this immature stage they are called nymphs or naiads and are aquatic. This nymph eats and grows, molting its exoskeleton, until its last molt, when what comes out is not another nymph but an adult insect. Dragon flies grow this way.
The second category is complete metamorphism also called homometabolous (whole change) metamorphisman. An adult lays an egg, from which hatches a larvae, (caterpillar, grub etc) which grows through several different stages, called instars, molting an exoskeleton or outer skin as it passes from one instar to the next. Then the larvae turns into a pupa, the stage of true metamorphosis, during which the cells of the insects body completely rearrange themselves. It is often referred to as a resting stage in the literature, but the insect is only appearing to rest. Inside there is a major transformation going on. This is the cocoon stage of the butterfly, as an example. From the pupa emerges the adult, totally and utterly reconfigured from the larvae.
Complete metamorphism is by far the most popular means of insect growth, found in approximately 10 times as many species as incomplete metamorphism. There are some distinct advantages to having such wide variation in the different stages of growth, and the first is division of labor. Ken Kaufman in his guide to insects refers to each stage of the insect as having a different job, and the analogy is a good one. The egg has it the easiest, it simply has to incubate. The larvae are “eating machines”, their job is to get as much nutrition as they can because the metamorphosis that occurs in the pupa stage is so incredibly hard and energy consuming. The adults may or may not eat, but their main job is to reproduce. Typically the larvae also live in different habitats than the adults (sometimes really different habitats, for example the larvae of black flies and mosquitos are totally aquatic, they live under water). This can reduce intergenerational competition for resources, always a good thing.
So it is larvae in the trees that is making all that frass, larvae doing all the work of eating, for many of these caterpillars turn into moths and butterflies that don’t eat at all. It’s a division of labor so stark it is hard to imagine. Its also perfectly synched to the rhythm of nature, those caterpillars, grown fat on summer’s greenery, will pupate and spend the long winter slowly rearranging their cells, preparing to emerge in the spring as new beings. It makes me feel crass and ill fitted in comparison. As the days grow shorter, we would do well follow their example, respecting the season and using what it offers, a break from the frenetic frass making of summer, to renew and reorganize ourselves as well.
www.bugguide.net The definitive internet insect resource, email in your photos to get identifications from an online community of bug nerds.
Ken Kaufman’s Field Guide to Insects of North America 2007 Houghton Mifflin, I love this book, it (like all Kaufman guides) uses actual photographs, but at the same time manages not to be too creepy.
Freeman, Quillin, Allison Biological Science 5th ed. 2014, Pearson
Most any modern biology textbook will give you some good basic information about Arthropods
Nice site from the West Virginia University Extention service, clarifying different types of metamorphosis,