Cicindelidae of Colorado
Jason P. Schmidt
Cicindela (tiger beetles) have life cycles that last from one to three years. The life cycles are named for the time of adult emergence (spring-fall or summer). All tiger beetles pass through three larval instars before becoming sexually mature adults. The first instar lasts between one and three weeks depending on biotic and abiotic factors. The second instar lasts from several weeks to one or two months. The third instar lasts from several months to over a year (Knisley and Pearson 1984)
An example of a spring-fall species of tiger beetle is C. purpurea. The eggs are laid in May and larvae reach the third instar in August of that summer. During the autumn, the tiger beetle goes into diapause. The diapause is broken in April. Immediately, the larvae feed then pupate in July. In the fall, a sexually immature adult emerges to feed and then goes into diapause for the winter. The following April, the sexually mature beetle emerges to mate and die. The life cycle lasts two years. The larval stage lasts from 12-13 months, and the adult stage lasts around 10 months (Shelford 1908).
An example of a summer species of tiger beetle is C. lepida. The eggs are laid in July and the second larval instar is reached by autumn. The larvae diapause until June or July when it will reach the third instar and feeds until autumn. The larvae diapause again until May and then pupates. The beetle emerges as an adult in late June or early July (depending on the rains) to mate and die. This life cycle lasts two years. 21 months are spent as larvae and two months are spent as an adult (Shelford 1908).
The larvae have large heads, prothoracic regions, and highly wrinkled abdomens. Larvae live in burrows constructed in a variety of substrates (Shelford 1908). Burrow length, form, and shape vary greatly between species and substrate. The burrows are cylindrical with a circular opening. The area around the burrow is smooth and round. However, a few species such as C. willistoni build turrets around their burrow entrance. Turrets are structures that are 1-4 cm above the burrow. It is hypothesized that the turrets are for thermoregulatory or prey attraction purposes (Knisley and Pearson 1981). Larvae acquire food by putting its head near the entrance of the burrow and waiting for prey to come within range. It uses its legs as well as specialized hooks on the dorsum of the 5th abdominal segment to anchor into its burrow (Pearson and Knisley 1985). This allows the larvae to catch the prey and not be pulled out of its burrow. Larvae feed on soft-bodied arthropods such as, flies, beetles, spiders, butterflies, and dragonflies (Shelford 1908).
Bombyliid flies and Tiphiid wasps are parasites of tiger beetles. Bombyliid flies in the genus Anthrax parasitize up to 70% of some populations of beetles (Knisley and Pearson 1984). The larvae of bee flies (Bombyliid flies) can parasitize tiger beetles during all three larval instars, but does not kill the beetle until it reaches the prepupal stage (Palmer 1982). The female fly drops her eggs in or around the edge of a larval burrow. The fly larvae hatch quickly and attach onto the beetle larvae. The beetle is allowed to develop into the prepupal stage before the fly larvae devours the internal organs of the beetle, killing it (Shelford 1913; Knisley and Pearson 1984). A female tiphiid wasp will deposit her egg directly onto the larvae after a paralyzing sting. When the egg hatches, the wasp larva consumes the tiger beetle larvae immediately (Mury Meyer 1983; Knisley and Pearson 1984).
Prey Acquisition and Predator Avoidance by Tiger Beetles (Coleoptera: Cicindelidae)
Adult tiger beetles, are visually guided, highly predacious Coleoptera. They have long, slender legs for running and are predominantly fossorial (Carter 1989; Lovari et. al. 1992). Cicindela are reported to be one of the fastest terrestrial arthropods on earth (Kamoun and Hogenhut 1996). Most Cicindela are diurnal heliotherms except while making dispersal flights. Diurnal behavior is most common because these beetles need to be at their upper lethal limit body temperature to have optimal physiological performance (Dreisig 1980). Most species, about 88%, of Cicindela are cryptically colored and elytral maculation varies greatly between species. However, some species have metallic and brightly colored elytra (Schultz 1986).
Cicindela hunt in habitat that is clear of visual obstructions and usually bordered by vegetation (Carter 1989). This preference in hunting habitat reflects the behavioral and physiological technique by which Cicindela hunt. Cicindela generally locate prey items with two behaviors: active foraging and ambushing. Cicindela actively forage by randomly searching open ground and catching anything that is small enough to eat (Swiecimski 1957). Some forest dwelling species such as Cicindela sexguttata, however may also use this method; in this case the beetles will search under objects such as leaf litter (Schultz 1998). Active foraging Cicindela do not usually see the prey item before it is contacted, thus it is thought to be perceived by chemoreception (Swiecimski 1957). Active foraging is not as common as ambush predation because of the thermodynamic constraints of the insects (Schultz 1998; Schultz 1992). During ambush predation, Cicindela choose an ambush site and wait for a prey item to come into view. At this time, prey is perceived visually. When prey is visually located, the Cicindela changes postural position to a more alert inclined posture, face the prey, then make a swift stop and go approach the prey item (Swiecimski 1957). Many species of tiger beetle are prone to change ambush sites frequently. These new chosen sites at a distance about 10-30cm and are equal to the tiger beetles range of vision. Moving foraging sites in this manor provides for a thorough coverage of the foraging area. If prey is encountered, foraging sites are chosen close to the encounter site. This behavior alteration may be indicative of memory (Gilbert 1986; Kaulbars and Freitag 1993).
It has been determined that visual recognition of a preys presence alone is not enough stimuli to elicit an attack. In addition to visual confirmation of the prey item, the quarry must have movement, and be of appropriate size and color to prompt further predatory behavior. Lovari et. al. (1992) did a series of experiments and determined some cues that elicit an attack or avoidance response in a European species of tiger beetle. The tiger beetles were presented with stationary or moving baits of varying size and color. The study determined that the most important visual cue to prompt an attack was the movement of the prey item. Out of 522 tests on stationary bait, only 3.83% were attacked in the absence of movement, where as 57.26% out of 2674 moving baits were attacked. Tests were then preformed to determine the effect of the size of a prey item on the predatory response. It was determined that tiger beetles were most responsive to prey that was about half of their body length, with smaller sized prey favored over large prey. A smaller prey size preference may occur due to increased energetic expenditures of killing larger prey. In addition, when larger prey is attacked, the possibility of injury or death increases. Therefore, attacking large prey may not be as beneficial to the predator as easily overcoming smaller prey. Finally, attacks provoked by certain colors were analyzed. This study provided evidence that colors with high contrast with respect to ground or base colors are the most frequently attacked colors (Lovari et. al. 1992).
After prey is detected and a predatory response is elicited, the prey is pursued. Tiger beetles capture their prey by rapid terrestrial pursuit. At no time during prey pursuit have Cicindela been documented to fly after prey that makes a winged escape. For this reason, prey pursuit is said to be two-dimensional (Kaulbars and Freitag 1993). Visual acuity and high terrestrial velocities make Cicindela efficient predators in open habitats. However, high angular speed proportionally degrades images by overloading the sensory system with images (Land 1997; Gilbert 1997). Predators can overcome this phenomenon in one of two ways: either by using stealth to get close to their prey followed by a shortened chase, as seen in most large mammalian hunters and herptiles. Or they can relieve and readjust their sensory system by closing their eyes or pausing as seen in cheetahs, some fish and birds (Pough et. al. 1999). Cicindela start out by ambushing their prey but usually break cover early, thus leading to a longer chase and more room for visual error. Tiger beetles do not have eyelids, thus briefly closing the eyes is not an option for these insects. Consequently, tiger beetles adopt a stop-and-go running pattern. While stopped, the angular velocity of the tiger beetle does not compound the blurred appearance of the moving prey. As a result, during the chase the beetle stops, but only long enough to relocate the prey. It has been determined that the length of the stops, and thus the length it takes for the beetle to relocate the prey, is determined by the angular velocity of the prey. If the prey moves at a relatively slow angular velocity, the tiger beetle has to pause longer to relocate the prey than if the prey moves at a rapid angular velocity (Gilbert 1997). This may be counter-intuitive, but due to the structure of compound eyes, this system can be explained. Compound eyes are made up of several individual ommatidia. Each ommatidium is an elongate visual receptor (Borror et. al. 1989). Considering the structure of the eyes, slower moving insects take longer to move from one ommatidial field of vision to the next. Therefore, the tiger beetle has to wait longer for slower moving insects to be relocated. After the prey is relocated the tiger beetle will make any angle of attack adjustments necessary to continue pursuit and another short run begins (Gilbert 1997).
Cicindela have evolved many anti-predator mechanisms. These mechanisms include structural color, evasive flight response to sound, chemical defense, rapid terrestrial locomotion, flight, body size, and gregariousness. Explanations for having one organism with so many anti-predator mechanisms come form David Pearson. Pearson (1989; 1985) suggests that a prey item may have to overcome many different predators that focus on different cues. In this context, where one mechanism, such as crypsis, may be effective for birds as a predator, another mechanism, such as defensive chemicals, may be required to ward off other predatory insects. More than one anti-predator mechanism may be required if a predator can overcome the first mechanisms encountered. It has also been noted that in this situation the prey may present increasingly potent defenses if the initial less potent defenses are overcome.
Attacks on animals can be broken up into sections, which include a search phase, pursuit phase, capture phase, and a processing phase. In this case, the prey item, tiger beetle, may have evolved specific defenses uniquely targeted towards each of these phases (Pearson 1989, 1985).
While a predator is in the searching phase, a tiger beetles best defense may be structural coloration and accompanying behavior. Crypsis in tiger beetles is apparent in dull-colored maculate or immaculate beetles. Solid colored tiger beetles may be cryptic because they resemble dark or light substrate, or resemble objects (such as pebbles or debris) in contrast to the substrate. Behaviorally, this coloration is justified by the short jogs and frequent pauses of a tiger beetle during ambush predation or thermoregulatory behavior. Some beetles are solidly colored but do not resemble either the background color or an object on the surface, but may resemble the other surrounding patterns while in flight (Schultz 1986). Uniform coloration and sedentary behavior comes at a price for the tiger beetles in the form of thermoregulation (Hadley et. al. 1992). Maculate individuals may appear more cryptic in multi-colored environments because the maculation breaks up the form outline. Whether the beetle is solid or maculate, the threat of changing backgrounds must be dealt with. Since a tiger beetle cannot change its color, it must change its behavior to accomplish this. One example of changed behavior is the beetle occupies a habitat that is highly cryptic during the time predation is a threat, and only switches backgrounds when the threat is reduced (Schultz 1986). This is exemplified by Cicindela repanda. During the day this dark brown species of tiger beetle can be commonly found by wet mud. Here the beetle is cryptic, but in the evening it will find drier soil to burrow and remain for the night. In this case the beetle was on the background color that optimally matched its color during the peak of predator threat. At night when the threat of predation was less, C. repanda switched to lighter substrate to overnight (Carter 1989; Schultz 1986).
88% of North American Cicindela are cryptically colored (Willis 1967). The remaining 22% are brightly colored and usually iridescent or metallic conspicuous beetles. Iridescent coloration can be a highly effective anti-predator tactic. Dull cryptic colors reflect a wide range of light waves, thus the hue stays at a relatively constant color regardless of the angle observed. On the contrary, bright reflective colors reflect a narrow range of light wavelengths, thus have the capacity to change color dramatically at different angles of view and become dark under low light intensities. This property can be useful for predator evasion because of the spontaneity of the color. As a predator approaches an iridescent beetle, even if the beetle dose not change position, the color of the beetle will change as the predator changes its angle of view. The iridescence and its associated properties can also serve as flash colorations. If a predator approaches a basking, iridescent tiger beetle; the beetle will either change position or fly away. If the beetle chooses to change position it will usually change to an area with lower light levels, thus becoming dark and blending with its surroundings. This effect may be exaggerated because the predator initially encountered a brightly colored object and then the object turns dark, giving the illusion of disappearance. If the tiger beetle takes flight, the predator encounters the same dramatic change in color, but the contrast may be even further compounded with changes in flight angles (Acorn 1988; Schultz 1986).
Kamoun (1991) suggested that Cicindela might exhibit parasematic coloration. It was hypothesized that dorsal elytral spots on some species of tiger beetles may serve as eyespots or deflectors. If the color spots do serve this function, they help ward off potential predators or serve to deflect the attack onto a less crucial part of the body (Kamoun 1991).
Aposematic coloration in tiger beetles may be present. Several species have orange or other brightly colored abdomens. This form of coloration is thought to warn potential predators that the prey item is unpalatable or toxic. All tiger beetles posses defense glands and release benzaldehyde or a derivative of it when agitated (Pearson 1989).
During the next phase of predation, the pursuit phase, more evasive actions may be taken. Cicindela usually fly only when being chased by a predator. During this time flights are short and fast (Nachtigall 1996). However, during distributional flights at night, tiger beetles are susceptible to aerial predation from bats. Studies by Spangler (1988) and Yager et. al. (2000) show that tiger beetles react behaviorally to ultrasonic pulses that mimic bat frequency. The common evasive behavior is to turn the head to the opposite side of the signal, push the middle legs to the opposite side, decrease the angle of the elytra, and increase the hind wing beat. This change in behavior causes the tiger beetle to move away and down from the signal. The tiger beetles respond with their own ultrasonic clicking to confuse the bat (Yager et. al. 1997; Spangler 1988). Some species do not show any reaction to sound pulses. However, it was found that there is a strong correlation with seasonal activity and the propensity to react to sound stimuli. Species active in the spring and fall do not show a reaction to ultrasound, whereas species that are active in the summer do. It was also noted that some species are attracted to lights at night while others are not. This event may be part of the explanation for the loss of ultrasound sensitivity is some species. Some of the species that were not attracted to lights were also spring-fall species. If these species are not active at night the need for ultrasound detection may be reduced (Yager et. al. 2000).
Tiger beetles may evade the third and fourth phase of predation, capture and processing with the use of defensive chemicals. Tiger beetles have been found to produce several defensive chemicals in the pygidial reservoir. The main chemicals found were mandelonitrile, hydrogen cyanide, and benzaldehyde. Benzaldehyde is the main defensive chemical in the defense of tiger beetles (Blum et. al. 1981). Benzaldehyde is a highly volitle aldehyde that diffuses readily into the surrounding air, thus is easily transmitted to antennae and olfactory systems of potential predators. Benzaldehyde is considered a nonspecific irritant. If provoked, a tiger beetle secretes either a dark fluid or a clear gaseous secretion of benzaldehyde form the pygidial gland. This secretion is not forcefully ejected from the posterior end, but wicks out and dissipates to the surrounding air. This is accompanied with the behavior of bending the tip of the abdomen. Aldehydes are thought to deter invertebrate predators by blocking chemoreception, but have little to no deterring influence on vertebrate predators (Kelley and Schilling 1998). Hydrogen cyanide is toxic to vertebrate predators but is not excreted during confrontations. Therefore, it is thought that vertebrate predators are deterred by benzaldehyde because it may be seen as an aposematic chemical signal of hydrogen cyanide (Blum et. al. 1981). In the processing phase chemicals are also used to encourage the release of the tiger beetles. When captured Cicindela punctulata and other Cicindela will excrete dark extradigestive juices. The enzymes in these oral secretions may function to impede the processing phase (Pearson 1989).
Gregariousness may serve to enhance chemical defenses. Kelley (1998) observed that on a particular day benzaldehyde could be smelled near a large aggregation of Cicindela repanda (Kelley and Schilling 1998). Pearson (1989) proposes that tiger beetles in large aggregations could optimize the effects of chemical defenses by being around other tiger beetles. Instead of an individual secretion the whole aggregations secretions are being exuded thus the effects are amplified (Pearson 1989).
Pearson (1989) also noted that upon capture, a preys body size may help to deter further predatory behavior. Larger tiger beetles were not preyed upon as much by other insects and herptiles as much as small ones. However, the trade-off is that small insects were preyed upon less by birds than large ones (Pearson 1989). As seen earlier, body size effects predation by tiger beetles on other insects. In the form of energetics, preying on larger organisms may consume more energy than what is gained, thus being of an non-advantageous nature. In addition, injury or death to the predator could accompany the attack (Lovari et. al. 1992).