Stonefly

A small order of insects hemimetabolous in nature, stoneflies are any of the 3500 described species, still extant, belonging to the order Plecoptera. Stoneflies are present worldwide with the exception of the continent of Antarctica and new species of the order are still being discovered. Plecoptera insects are found in the Northern as well as the Southern hemisphere. From an evolutionary standpoint, stoneflies are believed to be among the most primitive Neopteran groups. Among the Neotropical stonefly fauna known, there have been found to be more than 500 species of stoneflies categorised under 6 families.

In this article, we will attempt to learn about stoneflies in great detail including their morphology, life cycle, reproduction and more.

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An image of a stonefly is shown above

Stonefly Classification

Stoneflies refer to any of the wide numbers of species included in the order Plecoptera. As such, the scientific classification of stoneflies is: 

• Kingdom: Animalia

• Phylum: Arthropoda

• Class: Insecta

• Order: Plecoptera

Stonefly Description 

Morphology of Stoneflies - The adults of stoneflies possess long antennae and weak, chewing mouthparts along with 2 pairs of membranous wings. The size of stoneflies varies between 6 and 60 mm (0.25 and 2.5 inches). The hindwings of stoneflies are usually larger and shorter as compared to the forewings. When not in use, the hindwings fold into the shape of a fan. Despite having well-developed wings, stoneflies are poor fliers.

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Fig: Stonefly Morphological Depiction

The body of stoneflies is divided into three general regions:

1. Head

2. Thorax

3. Abdomen

Head - The head region of stoneflies comprise stout antennae and compound eyes, two each along with three ocelli in general. Contrasting pigments found in a number of giant stonefly species predatory in nature form definitive patterns. These pigments usually form a ‘W’ or ‘M’ across the head of these giant stonefly species. The head region of stoneflies consists of the mouthparts of the chewing type which can also show adaptation to detritivorous or predatory lifestyle. Long teeth are found in the predatory stoneflies enabling them to stab and macerate their prey. Such stoneflies, however, consume their prey whole. On the other hand, the detritivorous stoneflies make use of scraper-like structures for the consumption of leaf particles and the accompanying biofilms. In most of the stonefly families, maxillae and mandibles are functional in adults, barring the usually vestigial mouthparts of some of the giant predatory species. 

Thorax - Three segments comprise the thorax region of the stoneflies. From each of the thoracic segments, legs arise. The legs are further composed of coxa, femur, trochanter, tibia and a 3-segmented tarsus. The fringes of the tibiae and femora often contain hair fringes and assist in swimming. The gills found on the thorax region of the stoneflies vary according to the family and maybe blade-like, branched or finger-like. 

Abdomen - The abdominal region of the stoneflies is relatively softer. This region is also found to include nymphal gill remnants even when the flies are adults. As in the case of most stoneflies, 10 external segments that are easily discernible are present in this region. In the abdomen of nymphs, a continuous fringe on the outer surface is often formed by hairs. A female can be identified if this fringe is medially disrupted on the 8th sternum. In a few families, clustered or finger-like, filamentous gills are present in the abdominal region. In the nymphs,  2 multisegmented, stout cerci always project from the 10th abdominal segment. Nymphs and adults both have paired, long cerci that project from the tip of their abdomen.

Prominent Stonefly Genera

• Common Stonefly: These stoneflies belong to the family Perlidae. The adults of common stoneflies are usually olive-brown or dark brown in colour. The wings of the common stoneflies are engulfed in dark veins and the wings overlap one another. The lower part of the head as well as the antennae’s base are yellow in colour. Additionally, two yellow spots one adjacent to each shoulder are also present between the wing and the neck region. These stoneflies are most active during summertime. 

• Winter Stonefly: The stoneflies of the order Capniidae with 10 Genera including 157 Species. These stoneflies grow and mature in the coldest seasons during the year, hence the name. The aquatic larvae of the winter stoneflies spend their lives in streams and rivers. These larvae also highly depend on oxygenated water as a means of survival. The ability of winter stoneflies to withstand and thrive in colder temperatures allows them to take advantage of improved habitat conditions when other aquatic insects are mostly absent. 

• Yellow Stonefly: The stoneflies of the genus Isoperla are commonly referred to as yellow or little yellow stoneflies. These stoneflies are commonly found in trout streams. There are more than 60 species of yellow stoneflies. 

• Green Stonefly: The green stoneflies are those belonging to the family Chloroperlidae. There are more than 22 genera and 200 species included in this family. The wings of these flies are distinctly green in colour, and hence their name. The green stoneflies are found in the benthic zone of the cold rivers and streams and are widely distributed among five continents. The green stoneflies are also commonly used by fishermen for trout fishing. 

The stonefly genus with the most diversity in the Neotropical region is Anacroneuria with 348 valid species described. This genus is distributed widely ranging from Argentina to the southern United States. The stonefly genus Enderleina comprises 6 species. These species are prominently found in northern Brazil and southern Venezuela with one species identified from northern Venezuela. 38 species have been identified from the genus Kempnyia and these are found only in Brazil. Macrogynoplax, another prominent stonefly genus has 15 species described. These species range in their distribution from Guyana, Peru, Venezuela, Brazil, Surinam to Colombia.

Reproduction and Life-cycle of Stoneflies

The female stoneflies can lay up to a thousand eggs at a time. The eggs are generally dropped in the water by the female stoneflies flying over. Alternatively, the eggs of stoneflies may also hang on a branch or a rock. The sticky coating present on the eggs aids them in adhering to the surface of the rocks thus minimising the risks of swift currents sweeping them away. Once adhered, the hatching time of the eggs vary between two and three weeks. However, certain species of stoneflies also exhibit diapause where the eggs are in the dormant stage and remain so for the entire duration of a dry season. Such eggs hatch only when the conditions return to normal.

The life cycle of stoneflies is slow and seasonal. The nymphs of stoneflies require anywhere between 1 and 4 years to complete development, depending on the species. During the transition from the nymphal to the adult stage, the flies undergo 12 to 36 molts up until they emerge and become terrestrial in the adult stage. The nymphs, before becoming adults, leave the water surface, become attached to a fixed surface and undergo molting for one last time. 

Darkened wing pads are indicative of the transformation of nymphs to adults in the late instar nymphs. In a few species, the adults lack wings and are termed apterous, while the wings are short in brachypterous species. In the case of most species, the wings are fully developed and are termed macropterous. Utacapnia tahoensis, a capniid species, lacks wings entirely and thus, complete the entirety of its life cycle in the deep waters of Lake Tahoe, Nevada and California, USA.

The life span of adult stoneflies generally lasts only a few weeks. The adults emerge only at specific time periods of the years when optimal resources are available. Usually, the adults stay near the lake or stream from where they hatched as they are not strong fliers. Some adults do not feed entirely and of those that they do, they exhibit herbivorous behaviour.

Diet of Stoneflies

Stoneflies can be abundantly found in rocky streams where they can represent up to 10 percent of the production by invertebrates. The stoneflies play important roles in the food webs of the stream. Stoneflies are adapted for herbivory as indicated by the anatomical differences in the mouthparts of the nymph or for predation. The functional feeding habit of stoneflies can vary as the nymph undergoes maturation. The stoneflies that exhibit a predatory lifestyle are mobile hunters rather than predators of the sit and wait type. The stoneflies maybe top invertebrate predators. The common prey of stoneflies include mayflies, caddisflies, midges and small stoneflies even. 

Symbionts and Parasites of Stoneflies

A number of organisms including midges, wasps, larval mites, nematodes, fungi and the bacterium Wolbachia parasitize on stoneflies. Chironomid midge Nanocladius have been found to parasitise Pteronarcys biloba nymphs in a stream in eastern Canada. The mites that are shifting to adults or larval mites often parasitise stonefly nymphs as they undergo molting. Fungi belonging to the order Harpellales have commonly been found to occur as symbionts in the gut of stoneflies. These fungi are thought to aid in the digestion of the decayed plant material consumed by certain species of stoneflies. Larger invertebrate predators such as fish and hellgrammites act as predators of stoneflies. 

Evolutionary History of Stoneflies

As one of the most primitive Neopteran groups, close relatives of stoneflies have been identified from the Lower Permian and carboniferous geological periods. True stoneflies, on the other hand, are identified from a bit younger fossils. The modern diversity of stoneflies is thought to have originated in the Mesozoic era. Although the populations of stoneflies present in the Northern and the Southern hemispheres are distinct,  evolutionary evidence indicates these populations are likely to have crossed the equator a number of times prior to becoming geographically isolated once again.  

Conservation of Stoneflies

Stoneflies, ecologically and numerically, are a pivotal component of the fauna of running waters. With the pollution in running waters increasing mainly due to industrial, agricultural runoffs and sewage, the population of stoneflies is getting affected. Water pollution results in habitat fragmentation in streams altering watercourses and banks causing many stonefly species, given their narrow ecological requirements, to become vulnerable and even threatened with extinction. Several species of European stoneflies have, in fact, become extinct while many others have been reduced to isolated small populations.

As in regards to the status of conservation, stoneflies are considered as most threatened as per the IUCN red data list while the number of species included in this list is less than what should be.

Stonefly as Bioindicators

Plecoptera insects are highly sensitive to pollution and habitat alteration. Stoneflies are thus good bioindicators of the quality of the environment. Since a large number of stonefly species, the nymphs, in particular, are aquatic, they also serve as indicators of water quality along with caddfishes. Stoneflies lay their eggs in water and the larvae also hatch in water. These larvae require a greater concentration of dissolved oxygen in order to respire and are thus very sensitive to water pollution. So in waters with a low concentration of dissolved oxygen, such as those with algal blooms will not have populations of stoneflies.

Did You Know? 

• Stonefly is also an action and adventure game. The game promises a tranquil and chill adventure about self-discovery. The players are required to traverse through the wilderness of the place called Stonefly where they are confronted by angry birds, dangerous fauna among the beautiful flora in the game.  

• There is a firm providing data centre solutions known as Stonefly Inc. They provide elastic, high-performing IT infrastructure that is always available. They are known for supporting data-dependent applications and processes seamlessly at any time and place. 

So, in conclusion, we can say that Plecoptera, commonly known as stoneflies, is a small but highly diverse order of hemimetabolous insects, given that 3500 and more species have been described worldwide to date. These insects and their nymphs act as bio quality indicators, especially in aquatic ecosystems.