A “chokepoint” is an important strategic passage through an otherwise impassible territory, usually substantially narrower than that territory, which greatly decreases the advantage of a superior attacker. In a military context, King Leonidas I’s leadership of the Spartans to defend the narrow Pass of Thermopylae during the Persian invasion of Greece, led by Xerxes I on 21 August 480 BCE, serves as an historic example. “Phragmosis” (from φράγμα, pronounced “frágma,” meaning “barrier” or “shield”) is a similar evolutionary strategy that integrates a specialized morphology (shield) and a corresponding behavior to defend a narrow opening (chokepoint) by a single individual or small group of individuals, described originally for the structure and behavior of certain species of soil-nest-building, colonial ants (Wheeler 1927). When an animal lives in a protective nest, an opening for egress and ingress is a vulnerable threshold for potential attack by a predator or parasite. One way to protect this threshold from intruders is for residents to evolve both a morphologically near-impenetrable surface (usually a hard, flat, thick body part, a “shield”) that fits the entryway and a behavior to use that surface for blocking the entrance when threatened. To be effective, there should be little if any structure on the shield that can be grasped by a predator, but there may be sharp spines, dense hair, or other features that can serve to help deter or repel the intruder. In insects, usually the head (with thoracic nota sometimes included) or the end of the abdomen evolved an especially hardened area that is typically truncate and with edges that fit the entrance of their nest and that the insects use to block the entrance of the nest. We propose to expand this concept of phragmosis to include structures and behaviors to block the entrance of a portable and protective case or shell. Does this phenomenon occur in at least some case-bearing caddisflies?

Alternatively, some encased animals defend the chokepoint opening of a portable case or shell by pressing it against impenetrable substrate. Such a case or shell has only one opening and the shape of that opening conforms to the shape of the impenetrable substrate. The encased animal may also have adaptations that help it to hold itself especially tightly against the substrate. We term this structure and behavior “cathaptosis” (from καθάπτω, pronounced katháptō, meaning to fasten or make fast) to describe the behavior of grasping the substrate firmly and holding it tightly against the shell or case opening. Does this phenomenon also occur in at least some case-bearing caddisflies?

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).

For the experiment to test for phragmosis when disturbed by a stonefly or hellgrammite predator, each of the larvae of F. pele and G. calcarata quickly withdrew into its case to expose only the flattened front of the head (F. pele) or flattened top of the head and anterior nota (G. calcarata), behavioral responses consistent with phragmosis. This activity was recorded (Bishoff 2023a, 2023b, 2023c). The response was seen even when predators were absent; a small movement of the dish or even a shadow prompted the animal to retreat quickly into its defensive posture, presumably because these phenomena are clues to the presence of a predator.

For the experiment to test for cathaptosis, an Ascalaphomerus specimen responded to the probing activity by gripping the piece of wood tightly and pressing the case opening onto the wood. The larva also spun silk to anchor the lower edge of the case even more securely to the wood. This activity and the silk anchor were recorded (Peng et al. 2023).

Recent studies have demonstrated that flat-headed structure in some Drusus species (Limnephilidae) is adapted to the creation of vortices for entrapping prey (Waringer et al. 2015, 2021) and is also associated with the use of filtering bristles for predation (Pauls et al. 2008; Vitecek et al. 2020) The musculature of these flattened larval heads is very similar to that of larvae of other Drusus species with normal, convex heads and lacking filtering bristles (Zittra et al. 2022). Drusus muelleriMcLachlan 1868 is a species that has long spines on the lateral edges of the head (Graf et al. 2005) that apparently help dissipate the force of the water, relieving hydraulic stress. These sharp points could also help to deter predators that might attempt to remove them from their cases, as may also be the defensive purpose of marginal elytral spines in some ambrosia beetles. We propose that this flat-headed morphology that has been associated with filtering predation or hydraulic adaptation may also be a structural component of phragmotic defense. It seems likely that larvae of D. muelleri and related species are using their flat heads not only to capture prey or maintain position in fast-flowing water, but also to protect themselves from predators, or a combination of these functions. It is unclear whether one function is a pre-adaptation of the other and, if so, the evolutionary direction of any pre-adaptation.

However, larvae with flat heads also live in a wide variety of other habitats and feed with various other methods on a wide range of food resources. For example, caddisflies with flat, apparently phragmotic structure inhabit not only fast-flowing water, but also organic muck, drifting sand, hygropetric habitats, or quiet pools with accumulations of organic debris (Table 1). Furthermore, they feed in ways other than predation, including shredding large pieces of detritus, scraping periphyton from hard and stable substrates, and gathering or filtering small detritus particles (Table 1).

Interestingly, there are also examples of flattened heads in some retreat makers of the suborder Annulipalpia. The larvae of at least some species of Macrostemum (Hydropsychidae) have flat heads that a larva may use to block the anterior opening of the side channel of its highly specialized retreat or to help direct the flow of water through its filternet, or both.

For the experiment to test for cathaptosis, the “case” of the Ascalaphomerus species is actually a small stick. Instead of assembling a case with silk and pieces of substrate, a larva of this genus gouges the pith from the axis of a single stick (or occasionally some other piece of wood), then transports the hollowed stick or piece of wood as though it were a case constructed of smaller pieces of substrate. The anterior opening of the case is beveled so that if it is in a cathaptotic position, it will be held at an angle to the substrate. During daytime, the substrate is usually a larger, dead tree limb lying on a stream bottom in slowly moving marginal water. When the Ascalaphomerus larva is gripping the dead limb in cathaptosis, its “case” appears to be a broken twig of the tree limb. Larvae of this genus typically remain still and cathaptotic during daylight hours, protected by camouflage from visual hunters. At night they moved more freely, shredding dead plant debris or capturing small animals for food.

In Trichoptera, potential examples of phragmosis (Table 1) seem to be more common than of cathaptosis (Table 2).

For an animal that lives in a burrow, nest, or portable case or shell, the threshold between their home and the outside world is a transition portal of vital importance. It is the animal’s point of departure from a relatively safe haven to access other vital resources. This vulnerability has served as selective pressure, resulting in the evolution of the defense techniques of phragmosis and cathaptosis. Both techniques seal the opening, but in very different ways.

We have demonstrated the behavioral components of these defensive behaviors for three species and provided lists of some species whose larvae and cases meet their respective, necessary structural criteria. We encourage colleagues to test these hypotheses with the species listed in Tables 1 and 2 and with any other species appearing to meet the structural requirements of these defensive strategies to determine whether the associated behavioral components also occur.

Conceptualization: MJB, JCM. Literature review: MJB. Investigation: MJB, LP, H-mZ. Original draft: MJB. Review/editing: MJB, LP, H-mZ, JCM. Supervision: JCM.

The authors declare no conflicts of interest. No other employees of our universities had a role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.