Ranatra

Fabricius, 1790

water stick-insect, water scorpion, water mantis

Species Guides

10

Ranatra is a of slender aquatic predatory insects in the Nepidae, commonly called water stick-insects or water scorpions. The genus contains over 140 distributed across freshwater worldwide, with highest diversity in South America and Asia. These insects are sit-and-wait that use forelegs to capture prey. They possess a distinctive long caudal siphon for breathing while submerged.

Ranatra drakei by (c) Matthew Pintar, some rights reserved (CC BY), uploaded by Matthew Pintar. Used under a CC-BY license.Ranatra montezuma by (c) Matthew Pintar, some rights reserved (CC BY), uploaded by Matthew Pintar. Used under a CC-BY license.Ranatra nigra by (c) Matthew Pintar, some rights reserved (CC BY), uploaded by Matthew Pintar. Used under a CC-BY license.

Pronunciation

How to pronounce Ranatra: /ræˈneɪtrə/

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Key distinguishing features include the extremely slender, stick-like body (more elongate than other water scorpions), long caudal siphon approximately equal to body length, and forelegs held in a grasping posture. When at rest among vegetation, typically positioned -down with forelegs extended. Distinguished from the Asian-Australian Cercotmetus by the longer siphon. Australian genera Austronepa and Goondnomdanepa are geographically separated.

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Appearance

Elongated, stick-like body typically 2–6 cm in length, with females averaging larger than males. Coloration generally brown. Wings present and functional for . Caudal siphon (breathing tube) at end ranges from less than half body length to slightly longer than body. Forelegs modified as grasping appendages with strong claws. Body form extremely slender compared to other Nepidae.

Habitat

Primarily inhabits stagnant or slow-moving freshwater including ponds, marshes, canals, and streams. Exceptional records from hypersaline lakes and brackish lagoons. Typically found among aquatic vegetation where they can anchor and ambush prey. Lentic waters are preferred across the .

Distribution

in freshwater worldwide. Highest in South America (approximately 50 ) and Asia (approximately 30 species). Additional species occur in Africa, North America, Australia (3 species), and the Palearctic region including Europe (R. linearis).

Seasonality

active year-round in suitable conditions, with activity reduced in extreme cold. Reproductive timing varies by region; hatch in 2–4 weeks and nymphs mature in approximately two months.

Diet

: captures and consumes other aquatic insects, tadpoles, and small fish. Prey is pierced with the and injected with saliva that sedates and externally digests the victim. At least one has been observed swimming in open water at night to capture zooplankton.

Life Cycle

deposited on aquatic plants just below water surface, or in mud in some . Incubation typically 2–4 weeks. Nymphal development approximately two months. Post-moult in nymphs includes specific foreleg movements (slow claw opening/closing, movements toward ) during the approximately 4-hour period between moulting and darkening. Winged capable of .

Behavior

Sit-and-wait that positions -down among vegetation with forelegs extended to ambush passing prey. Foreleg posture varies with hunger state: emptier correlates with forelegs bent backward toward head and spread wider apart, increasing responsiveness to prey stimuli. Post-moult nymphs exhibit three behavioral phases with specific foreleg movement patterns; presence of prey reduces duration of phases 1 and 3 and increases rates of movement during phase 3. Some swim in open water at night.

Ecological Role

in freshwater aquatic , controlling of smaller aquatic and vertebrates. Serves as prey for larger aquatic predators including fish and other invertebrates.

Human Relevance

No significant direct economic impact. Occasionally encountered by aquatic biologists and naturalists. May be present in fish ponds and ornamental water features. Subject of behavioral and physiological research due to specialized predatory adaptations.

Similar Taxa

  • CercotmetusOverlapping distribution in Asia to New Guinea; distinguished by distinctly shorter caudal siphon
  • AustronepaAustralian in same ; geographically separated from most Ranatra
  • GoondnomdanepaAustralian in same ; geographically separated from most Ranatra
  • NepaSame but with flattened, oval body form rather than slender stick-like shape; shorter siphon

More Details

Post-moult behavior

Nymphs exhibit distinct behavioral phases during the 4-hour period between moulting and darkening: phase 1 (approximately 25 minutes) with claws usually closed; phase 2 (approximately 25 minutes) with claws open and forelegs stretched forward; phase 3 (approximately 205 minutes) with increased foreleg movement. Older nymphs show more movement during phase 3. Presence of prey accelerates progression through phases and increases movement rates.

Foreleg posture and feeding motivation

Foreleg position correlates with hunger state and responsiveness to prey. Hungrier individuals hold forelegs bent backward toward the with wider separation, and show more numerous responses to repeated prey stimuli. Visual-only stimuli (above forelegs) elicit fewer responses than stimuli engaging both visual and mechanoreceptors.

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