Land Shrimp Sighting (Western Europe): What is it?

Question

This approx. 6cm long insect was moving rapidly across the asphalt in a wavy/jumpy motion. It makes me think of a shrimp. It has a kind of widened head like a hammerhead shark.

I got two shots with my smartphone camera. Not the best quality.

What is it?

• Position: Western Europe
• Season: Start of Summer (June 20)
• Time of day: Mid-morning
• Location: Next to a largish marsh area currently kept as mini natural reserve in the middle of agricultural fields after large-area school buildings got dropped on the landscape. Sadly this area is likely to be marred by additional construction soon, the money is just too good and attracts sharks and brown envelopes.

Answer 1

The size (~2 inches), broad "quadrate" head, and long posterior abdomen leads me to believe this is a relatively large larva of a Predaceous Diving Beetle (family Dytiscidae).

• The larvae are sometimes called "water tigers" due to their large mandibles and voracity as aquatic predators.

One possible example:

^{Dytiscus marginalis; Sources: Left: Wikipedia, right: Wikimedia}

From Wikipedia:

When still in larval form, the beetles vary in size from about 1 to 5 cm (0.5 to 2.0 in). The larval bodies are shaped like crescents, with the tail long and covered with thin hairs. Six legs protrude from along the thorax, which also sports the same thin hairs. The head is flat and square, with a pair of long, large pincers.

The larvae of these insects are found across the globe in most surface waters (lentic and lotic), and they are known to leave the water to dig a suitable terrestrial site in the mud to pupate (Miller & Bergsten, 2016) -- this is likely why you saw this fairly large individual on land near your marsh.

Distinguishing species:

Your image does not provide enough detail to convincingly distinguish to species. I will provide some general details below, but I'll also mention that many species are only differentiated by internal or microscopic differences (e.g., male genitalia; Miller & Bergsten, 2016), and that even more macroscopic differentiating details tend to be on the ventral side of the insect (which is not visible in your image).

• I couldn't find any accessible keys for the larvae of European or even more broadly Palearctic species.

  • Larval keys do exist, though just for other regions of the world (e.g., Michat et al 2008 for Argentina and Watts 2002 for Australia) or for only narrow taxa (e.g., Fery 2003 or Holmgren et al 2016).

  • Pederzani (1995) provides a global key for adults.

  • Miller and Bergsten (2016) supposedly provide a fairly stellar global-focused key to genera, but I could not find access to it online.

• Some characteristics of note:

  • Larval characteristics of the family (from WV DEP): two claws on each leg; legs 5-segmented; abdomen usually terminates in a pair of urogomphi

  • Differentiating characteristics between common genera Dytiscus and Cybister available on bugguide.net

You can find some similar looking species here, and -- although from USA -- this Dytiscus species shares similar dorsal paired/longitudinal striping, number of body segments, and similar caudal anatomy.

Certainly, the similar-sized (up to 60mm), double-striped, European species, Dytiscus marginalis (the Great Diving Beetle), that I show above could very well be your species.

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Citations:

^{Fery, H., 2003. Dytiscidae: V. Taxonomic and distributional notes on Hygrotus Stephens, with emphasis on the Chinese fauna and a key to the Palearctic species (Coleoptera). Water beetles of China, 3, pp.133-193.}

^{Holmgren, S., Angus, R., Jia, F., Chen, Z.N. and Bergsten, J., 2016. Resolving the taxonomic conundrum in Graphoderus of the east Palearctic with a key to all species (Coleoptera, Dytiscidae). ZooKeys, (574), p.113.}

^{Michat, M.C., Archangelsky, M. and Bachmann, A.O., 2008. Generic keys for the identification of larval Dytiscidae from Argentina (Coleoptera: Adephaga). Revista de la Sociedad Entomológica Argentina, 67(3-4), pp.17-36.}

^{Miller, K.B. and Bergsten, J., 2016. Diving beetles of the world: Systematics and biology of the Dytiscidae. JHU Press.}

^{Pederzani, F. 1995. Keys to the identification of the genera and subgenera of adult Dytiscidae (sensu lato) of the world (Coleoptera Dytiscidae). Atti della Accademia Roveretana degli Agiati, Serie 7 B, 4: 5-83.}

^{Watts, C.H.S., 2002. Checklists & Guides to the Identification, to Genus, of Adult & Larval Australian Water Beetles of the Families Dytiscidae. Noteridae, Hygrobiidae, Haliplidae, Gyrinidae, Hydraenidae and the SuperfamilyHydrophiloidea (Insecta: Coleoptera)., Cooperative Research Centre for Freshwater Ecology, Identification and Ecology Guide, 43.}