Life history of tiger beetles, Cicindelidae 



The intimate adjustments of a species to its 

 habitat and the manner in which it selects a particular 

 stage in the sere may be illustrated by briefly describ- 

 ing the life-histories of tiger beetles. 



Adult tiger beetles are bright-colored, alert, swift 

 fliers. They are frequenters of bare ground. Both 

 adults and larvae feed predatorily on ants, sowbugs, 

 centipedes, spiders, beetles, flies, dragonflies, butter- 

 flies, and larvae of various forms. Tigei beetles com- 

 monly dig shallow burrows in the soil for shelter. 

 They reach sexual maturity after several warm days 

 in spring or early summer after they have emerged 

 from hibernation. They copulate on warm, humid 

 days when there is an abundance of food and sun- 

 light. After laying their eggs, they die. 



The female deposits one egg at a time, and lays 

 up to 50 in all, in small vertical holes, 7-10 mm deep, 

 which she makes with her ovipositor. The female 

 tests soil with her ovipositor until she locates soil of 

 the required characteristics. Hatching occurs in about 

 two weeks. 



The larvae are elongated, yellowish, and grub- 

 like. Anteriorly directed hooks, spines, and bristles 

 on the dorsal side of the larval body prevent the lar- 

 vae from being pulled out of their burrows by the 

 larger prey on which they feed. At the site of the 

 ovipositor hole the larva excavates a vertical cylin- 

 drical burrow 8-50 cm deep in temperate climates, 

 much deeper in colder northern regions. Most of the 

 time the larva stations itself at the top of its burrow 

 with its mandibles extended, and with its head and 

 prothorax just closing the round opening. It grabs 

 passing prey and carries it ofi to the bottom of its 

 burrow to devour it ; larger prey are eaten at the en- 

 trance. Inedible parts are cast out on the surface of 

 the ground around the burrow entrance. After feed- 

 ing 3-^ weeks, the larva closes the mouth of its bur- 

 row with soil and goes to the bottom to molt. 

 The second larval stage lasts five weeks or 

 longer, after which there is another molt. The 

 last of the larval stages closes the entrance to 

 its burrow in late August or September and goes to 

 the bottom to hibernate over winter (some species 

 hibernate in the second larval stage). The larva 

 comes out of hibernation in late spring and feeds 

 until summer. Then it closes the entrance of its bur- 

 row and constructs a side chamber in which it pu- 

 pates. The adult emerges in late summer and feeds 

 until October. It then digs a hole in which to hiber- 

 nate over winter. Two years are commonly required 

 to complete a generation, although in various species 

 the interval between successive generations may be 

 one to four years, depending in part on regional tem- 

 peratures. 



The niche requirements or serai stage preferred 

 by difl^erent species are rigid and appear determined. 



in large part, by the character of the type of soil a 

 species finds suitable for deposition of eggs and larval 

 growth. Studies performed under experimental con- 

 ditions demonstrate the nature of these requirements 

 (Table 8-4) but suggest no physiological explanation 

 (Shelford 1908, 1911, 1915: Balduf 1935). 



CLAY SERE 



Plant communities 



Erosion or calculated removal of overlying ma- 

 terial may leave bare areas of clay exposed. In clay 

 above pH 4.5 annual plants, of which smartweed is 

 particularly important, appear within a few weeks to 

 two years ; the higher the clay pH, the quicker the 

 appearance of vegetation. Within two to five years 

 thereafter sweet clover invades and develops nearly 

 complete dominance over large areas. Sweet clover 

 is a biennial, and an exotic species unimportant in the 

 sere in some parts of the country (Bramble and Ash- 

 ley 1955). Prior to its introduction, this stage in 

 the sere on bare clay may have consisted of the 

 perennial grasses still found in small scattered 

 patches, or it may not have been well developed. 

 A shrub stage seldom takes dominance over extensive 

 areas, but thickets of raspberries and blackberries, 

 smooth sumac, trumpet creeper, and various other 

 species succeed the sweet clover and grass stage. The 

 first trees begin to invade early in the sere, but they 

 are scattered and slow of growth, and do not attain 

 dominance for 25 to 30 years. The tree stage is com- 

 monly made up of eastern cottonwood, American 

 sycamore, silver maple, and American elm. Willows 

 occur in wet spots. Herb species of the first two plant 

 stages disappear, for the most part, in the shrub 

 stage. The herb stratum now consists largely of wood 

 nettle. Advanced forest stages of oaks, hickories, 

 basswood, and sugar maple will likely invade in the 

 future : as they occur now in adjacent areas. 



Ani 



i/ life 



The number of invertebrate species tends to in- 

 crease as the sere advances, although not always 

 regularly. In a study of a formerly strip-mined area 

 (Smith 1928), 18 species were found to be important 

 in the annual stage, 41 species in sweet clover, 40 

 species in shrubs, 32 species in the early forest stage, 

 and 67 species in the upland climax. More species 

 would be found in advanced stages because of the 

 greater variety of niches then available. Thus, in the 

 initial bare area there is only the ground stratum ; in 

 the annuals and sweet clover stages there are the 

 ground and herb strata ; in the shrub stage there are 



112 Habitats, communities, succession: 



