Anatomy of a Tiger Beetle Larva

My first experience looking for tiger beetles in Florida had gone well.  Despite its small size and urban surroundings, the narrow strip of coastal scrub and saltwater marsh along the intracoastal waterway behind my sister-in-law’s condominium boasted a robust population of what I took to be a single tiger beetle species.  The specimens I collected and photographs I took would later reveal that two co-occurring and closely related species were present: Ellipsoptera marginata (Margined Tiger Beetle) and E. hamata lacerata (Gulf Beach Tiger Beetle).  I had spent close to two hours under the August sun observing and photographing the beetles before I decided that I had given the preserve a thorough enough look.

As I was heading back, I noticed a little bit of high ground alongside a red mangrove thicket and went over to give it a look.  As I approached I saw something I hadn’t yet seen that day – tiger beetle larval burrows.  Larval burrows, especially larger ones such as these were, are unmistakeable – almost perfectly circular (slightly cut out on one edge) and smoothly beveled around the perimeter.  There were a number of burrows clustered on the small bit of high, dry ground, and my first thought was that their inhabitants represented the same (what I thought was a single) species that I had encountered so commonly that day as adults.  I then reasoned, however, that more likely they represented another species whose adults are active later in the season – perhaps one of the so-called “spring/fall” species whose larvae typically reach maturity during the heat of summer.  The size of the burrows (~5mm dia) suggested they were inhabited by 3rd instar larvae (the final instar before pupation), in which case it may be possible to rear a few to adulthood – if I could get at them. I tried fishing (Pearson and Vogler 2001) a few holes with a grass blade but didn’t get any bites, so I decided to watch for awhile and see if any of the larvae, believing the danger of my approach had passed, would reappear at the tops of their burrows.  Waiting for tiger beetle larvae to appear is a crap shoot – maybe they’re active, and maybe they’re not, and crouching in the stifling summer air of a coastal marsh in Florida is not an easy thing to do for very long.  Fortunately my wait was short, as within a few minutes I saw one re-appear at the top its burrow.  I slowly got out my knife and moved to place the tip on the soil about 1″ from the burrow at a 45° angle for an attempted tunnel block (Pearson and Vogler 2001), but it spooked and dropped back down into its burrow before I could get then knife in place.  No matter, I knew it was in there now and that it would likely reappear if I could muster the patience.  I positioned the knife and waited – crouched under the baking Florida sun, until when it did re-appear I plunged the knife into the soil with authority.  It was a good jab – I had blocked its retreat without injuring it, and a quick flip of the knife popped out the soil plug and exposed the startled larva, flipping vigorously in a vain attempt to escape before settling down amidst its unfamiliar, exposed surroundings.

For those of you who have never seen a tiger beetle larva, they are among the most other-wordly creatures one can imagine.  The large, heavily sclerotized head bears two long, sickle-shaped, upward-pointing mandibles and up to three pair of highly-acute eyes whose arrangement on each side conveys the image of a “face” with congenital birth defects.  The top of the head is flattened to lie flush with the surrounding soil as the larva sits at the top of its burrow, and huge, powerful mandibular muscles fill the cranial cavity.  The remainder of the body – long, narrow, and cylindrical – hangs from the head at a 90° angle down into the burrow and is unremarkably grub-like, save for a curious hump on the dorsal side of the 5th abdominal segment.  Close examination of the hump reveals an intricate pattern of forward-facing hooks and spines that function in anchoring the larva against the side of its burrow to prevent struggling prey from dislodging it.  The life of a tiger beetle larva is a life of waiting – unlike the adults who run down their prey, the larvae sit in their burrows and wait for prey to come to within lunge’s reach.  While the eyes of most grub-like insects detect little more than light and dark, those of tiger beetle larvae are densely packed with photoreceptors that permit detailed focusing and depth perception for detecting whether potential prey has ventured close enough to their burrow (Pearson et al. 2006).  When that happens, they strike with lightning speed, plunge their mandibles into their prey, and drag it down into the depths of their burrow where it is summarily dispatched with a few bites of their powerful mandibles.  Larvae consume they prey in a manner similar to that of adults in that they chew but don’t swallow their prey. Rather, they secrete digestive secretions containing proteolytic enzymes that begin digesting the prey extra-orally as they chew.  The resulting bolus is masticated and its liquid components sucked out until nothing but a dry wad of indigestable chitin remains, which is spat out of the burrow (Pearson and Vogler 2001).

Looking at this strange insect, it occurred to me that I had not yet attempted macrophotographs of a tiger beetle larva out of its burrow, and this would be a good opportunity to get more practice with my Canon MP-E 65 mm macro lens – a lens with incredible magnification capabilities, but one that is also a bit of a temperamental beast to use hand-held in the field.  The subject was unusually cooperative, perhaps too stunned by its sudden predicament to know what to do, and as I took the photographs I focused in particular on characters of the head and dorsal hump (often useful in identifying tiger beetle larvae, at least to genus).  Time was growing short once I finished taking photographs, so I placed the larva in a vial and returned the following day to extract a chunk of native soil to place in a rearing container, managing to collect two more larvae as well (unfortunately, one became instant “prey” for the other.  Note to self: when placing multiple tiger beetle larvae in a container of soil, seal the artificial burrows into which you place each one!).  I paid little further attention to the photographs, other than to transfer them onto my computer and add metadata upon my return to St. Louis.  I didn’t know what species the larvae represented, but I assumed they were something in the genus Cicindela or one of its several former subgenera.  However, had I studied the photos and considered the locality and habitat, I would have realized that my assumption was incorrect¹.  That realization would come in surprise fashion two months later when the two adults emerged within a few days of each other…

¹ Ten points to whoever can use this information to arrive at an identification before my next post 🙂

Photo details:
All photos: Canon EOS 50D, manual mode, ISO-100, 1/250 sec, MT-24EX flash w/ diffuser caps.
Photo 1: Canon 100mm macro lens, f/22, 1/4 power flash (photo slightly cropped).
Photos 2-4: Canon MP-E 65mm 1-5X macro lens f/16, 1/8 power flash.

REFERENCES:

Pearson, D. L., C. B. Knisley and C. J. Kazilek. 2006. A Field Guide to the Tiger Beetles of the United States and Canada. Oxford University Press, New York, 227 pp.

Pearson, D. L. and A. P. Vogler.  2001. Tiger Beetles: The Evolution, Ecology, and Diversity of the Cicindelids.  Cornell University Press, Ithaca, New York, 333 pp.

Copyright © Ted C. MacRae 2009.