Stonefly Mouthparts

David Nelson
First posted 4/13/2010 Last updated 5/28/2010

Insect mouthparts are quite interesting and, despite the variety of insect bodyplans, have a rather similar structure across most species. There are a bewildering number of insect species: at least 900,000 species are known (more than 80% of the species on the planet), and the estimates of the actual number ranges between 2 million and 30 million species. Despite this diversity, the structure of insect mouthparts is similar. While there is fascinating specialization in the mouthparts of some groups (the stylette of the biting flies, the coiled proboscis of the butterflies, the dueling mandibles of the stag beetles), overall the mouthparts are rather uniform, especially in the lower insects. Even the specialized insect mouthparts can be traced to modifications of the standard mouthparts.

In the generalized insect, there are four fundamental mouthparts (from superior to inferior): the labrum, mandibles, maxillae, and labium. An additional structure, the hypopharynx, is a midline, tongue-like structure between the maxillae.

The plectopera of North America have two basic mouthpart types, which correlate with their primary foodtype. Stoneflies with Type I mouthparts feed on detritus (dead plant and animal matter) and organic matter including diatoms, alge, and leaves that have been partly digested by fungus. They are termed “detritivores and herbivores”. Stoneflies with Type II mouthparts are carnivorous and feed on other aquatic insects, primarily midge larva but also on mayflies, caddisflies, hellgrammites, small fish, and other stoneflies. It would be naive to think that all stoneflies would neatly fit into these two simple classifications, and indeed there are intermediate types of mouthparts. Many species feed on both plant and animal matter. Many change over their lifetime, initially being a herbivore and changing to carnivory as they reach maturity. Interestingly, the structure of the mouthpart of these species changes over time, corresponding with their current food preference. It would also be naive to think that a species, even with characteristic Type I or Type II mouthparts, would only feed on one type of food. Plecoptera are opportunistic feeders and will vary their diet according to the availability of food items. There is an excellent discussion and summary of the literature regarding mouthparts and food habits in Stewart and Stark, 2002.

Labrum

The labrum is a small sclerite articulating with the lower part of the front of the insect face, the clypeus. It is a broad, flat lobe with muscles that allow it to extend or flex. The mandibles are visible just below the labrum. The labrum serves to hold food in place as it is chewed by the mandibles and thus can be described as functioning as an upper lip (“labrum” means “lip” in Latin). Type I mouthparts have a less-rectangular labrum than Type II mouthparts.

Mandibles

The mandibles are short, heavy, sclerotized (hardened), paired, unsegmented jaws just behind the labrum and in front of the cibarium. The cibarium is the upper end of the pharynx. The phyarynx in turn leads to the esophagus and the rest of the alimentary canal. The mandibles have two regions, a distal (apical) incisor region and a proximal molar region. They are complimentarily assymetrical so as to allow proper occulsion, ie, oppose each other in the midline and allow cutting or chewing of food into smaller pieces for swallowing. The shape of the incisor and molar regions varies with the diet. Carnivorous species have larger cusps in the incisor region. The mandibles have a well-developed system of muscles to allow for forceful chewing. There are strong adductor muscles (which bring the mandibles together) and comparatively weak abductor muscles (which separate the mandibles). The muscles attach in two ways. One muscle bridges between the mandibles and another attaches at one end to the mandible and at the other to the tentorium. The tentorium is a group of internal sclerites which forms the internal skeleton of the head. The mandibles have two strong joints; most of the other mouthparts have only one joint. Together these features correlate with the amount of force that is used to chew the food.

There are many variations in mandibular structure, including size of the apical “teeth” and the location and size of surface appendages, called bristles or hairs. All of the modifications are related to feeding activities, such as scraping diatoms from the surface of rocks, chewing apart of detritus or plants, or grasping and cutting of prey items. Many of these features are used in classification.

Maxillae

The maxillae are larger, segmented, paired structures just below the mandibles, and each bear a filaform (slender, filament-like) palp, which is a five-segmented organ for feeling, manipulating, and tasting the food. The first segment of the maxillae is the basal cardo (plueural cardines) with a single articulation with the head. The second segment of the maxilla is the stipes. The stipes bears at its apical end two structures, the lacinia and the galea. The lacinia are stout and triangular, with apical “teeth”. The lacinia are the most visible tooth-like structures when the mouthparts are viewed from the front or the bottom, and are particularly dramatic in the carnivorous Perlidae or Perlodidae. To the unfortunate chironomid, they must appear ferocious, indeed!

The dark "teeth" seen above are the lacinia of the mandibles

 

The galea of herbivores-detritivores are variously adapted for combing, raking, and tasting of food, with a variety of bristles, spines, and lobes, resembling in some ways a miniature form of baleen (both have a similar function). Some galea of herbivores have short but stout spines at their apex. The galea of predators have long, sharp spines at their tips for seizing and holding prey, along with the adjacent lacinia. The galea are the most inferior of all of the food-gathering mouthparts, and these adaptations are uniquely situated to facilitate the gathering of food, which of course is essential to the survival of the stonefly. The palp is borne on a lobe of the stipes called the palpifer. Overall, the maxillae are more complex in structure than the mandibles and serve a slightly different function. The mandibles are for chewing the food, the maxillae are for obtaining the food.

Labium

The labium (which also is Latin for “lip” but should not be confused with the labrum) is the functional lower lip. It is formed from the fusion of two sclerites into a single, midline structure behind or below the maxillae. It is divided by a transverse sulcus into a basal postmentum and a distal prementum. The basal postmentum is divided into a basal submentum and a distal mentum. The prementum bears a pair of filaform, three-segmented palps and a pair of two apical lobes, which together are called the lingula. The medial lobes of the lingual are the glossa and the lateral lobes are the paraglossa. The relative size of the glossa and paraglossa are important in identification at the family level. The labial palps are borne on lateral lobes of the prementum called the palpigers. The function of the labium is to assist in the manipulation and tasting of food.

(modified from Wikipedia, “Insect Mouthparts”; Borror, et al., 1989; Stewart and Stark, 2002; Chapman, 1982)

Figure 1: volar overall view of plectopera mouthparts

Frisonia pictipes (Perlodidae)

(drawn by D.Nelson; modified from Stewart & Oswood, 2006, page 197, figure 10.6D)

 

 

Figure 2: expanded volar view of plecoptera mouthparts

Frisonia pictipes (Perlodidae)

(drawn by D. Nelson; based on Stewart & Oswood, 2006, page 197, figure 10.6D; Stewart & Stark, 2002, pg. 89, figure 5.4, pag 90, figure 5.5; McCafferty 1981, pg. 21, fig 2.3; Borror, et al., 1989, sixth ed., pg. 39, fig 3-16; and Chapman, 1982, section 1.4.1.)

 


Figure 3: oblique volar view of plecoptera mouthparts

(drawn by D. Nelson; modified from Stewart & Oswood, 2006, page 197, figure 10.6D; McCafferty 1981, pg. 21, fig 2.3; Borror, et al., 1989, sixth ed., pg. 39, fig 3-16; Chapman, 1982, section 1.4.1; photographs from http://www.troutnut.com/hatch/53/Stonefly-Pteronarcyidae-Salmonflies, accessed in April of 2010; and photographs from collection of DNelson.)