Phragmosis, or the use of specially modified body parts and associated behaviors to block an opening as defense against predators, is a commonly observed phenomenon in certain ants and termites that block entrances of their subterranean nests with large, flat heads. It has been reported in some beetles and other insects and even in some frogs. Common features of phragmosis in caddisfly larvae include a hard and usually flat body surface, with or without stout spines, and the behavior of fitting that body surface tightly in the opening of its case. A different defensive strategy occurs in snails and case-making larvae of camptosomate leaf beetles (Chrysomelidae: Cryptocephalinae and Lamprosomatinae) that protect themselves from predators by securing the openings of their shells or cases firmly against the substrate, a behavior we call “cathaptosis.” Common features of cathaptosis in caddisfly larvae include a case with its vulnerable opening oriented parallel with the substrate and accompanied by behavior that grips the substrate, fixing the case opening firmly against it when threatened. We suggest that these defensive strategies have evolved multiple times in Trichoptera, especially in case-making larvae. We demonstrate some examples and provide tentative lists of caddisflies whose larvae may have evolved these defensive strategies.

To test our hypothesis for phragmosis, we observed the predator-prey interaction of case-making larvae of two Trichoptera species found locally in the Southern Appalachian Mountains when confronted with predators common in their habitats. The larva of Goera calcarata Banks, 1899 (Goeridae) makes a tubular case with lateral ballast stones for stability in fast-flowing water as it scrapes periphyton from the top surfaces of rocks in lotic-erosional habitats (Wiggins 1996). The larva of Fattigia pele (Ross, 1938) (Sericostomatidae) makes a case of fine sand and burrows in the sand of mountain springbrooks to gather fine organic particles (Wiggins 1996). 

Mature larvae of G. calcarata were collected from the tops of stones in a first-order stream at 34°45.366’N; 82°51.372’W, ca. 300 m a.s.l., and young larvae of F. pele were sifted from sand in a springbrook at 35°22.2’N; 83°6.6’W, ca. 1,520 m a.s.l. Available predators at those sites included larvae of Acroneuria abnormis (Newman, 1838) (Plecoptera: Perlidae) and Corydalus cornutus (Linnaeus, 1758) (Megaloptera: Corydalidae). Specimens of both the Trichoptera and the predators were transported back to the laboratory with stream water in separate plastic bags on ice and were kept alive and starved in refrigerated stream water until their interactions could be documented a few days later.           

The observation arena was a wide glass dish filled with stream water. A caddisfly and a predator were introduced into the arena and observed for a few minutes as they warmed to room temperature. We observed and recorded the interactions through a Celestron Microscope Pro® Model #44308 attached to a computer.       

To test our hypothesis of cathaptosis, we observed the case and behavior of a Chinese species of Ascalaphomerus Walker, 1852 (Calamoceratidae). Larvae of an Ascalaphomerus species were captured on pieces of wood in a pool of a mountain creek (P.R. China: Zhejiang Province, Li-shui City, Yun-he County, Dian-qing-shan Village, Yun-tan-xi Stream 28°9.72’N; 119°41.532’E, ca. 320 m, 9 August 2022) and brought with the wood to the laboratory for testing. Leaf litter, stones, and woody debris (twigs) were placed with the larvae in a rearing chamber. Instead of using a living predator, we gently agitated a test animal with a probe. The response of the larva was recorded with an Olympus TG-6 digital camera (Olympus, Beijing, China).