Insects that remain in diapause for more than 12 months or skip at least one breeding opportunity are in prolonged diapause (Hanski, 1988, Danks, 1992). Like seeds in a seed bank, prolonged diapause can provide a refuge from harsh environmental conditions and a means to hedge one’s bets in a variable environment (Slatkin, 1974, Beaumont et al., 2009). Compared to seed dormancy, prolonged diapause in insects is relatively rare. Unlike seeds, prolonged diapause of a cohort of insects tends to be finite, generally ending within three years, although some species remain in diapause for ten years or more (Hanski, 1988).

I recently demonstrated that Mormon crickets anabrus simplex (Orthoptera: Tettigoniidae) can require several years in the egg stage before they hatch (Srygley, 2020a, Srygley, 2020b). Like Old World katydids with prolonged diapause (Dean and Hartley, 1977, Ingrisch, 1986, Higaki and Ando, 2000, Ortis et al., 2023), new World Mormon cricket eggs have a facultative diapause stage in which they can remain for several years as a blastula (called ‘initial diapause’ or ‘egg diapause’, see the life cycle in Fig. 1). In addition, they have an obligate diapause stage (called ‘embryonic diapause’ because the embryo is nearly fully developed), which they enter to overwinter. After at least a month of subzero temperatures, embryos that are fully developed hatch the following spring or they do not survive. Mormon crickets are renowned for aggregating into large masses of millions of insects that move directionally through range- and croplands of the western United states (Wakeland, 1959), which suggests that development of eggs in the egg bank might be synchronized and makes Mormon crickets a useful and pertinent organism to study the cues required to break prolonged diapause

In this paper, I will focus on three measures of time: 1) counting the number of annual cycles, 2) chill time, and 3) duration of the warm period. In support of the first, Dean and Hartley (1977) conclude that some mechanism for counting the number of high or low temperature events is operating in the egg of the Old World tettigoniid Ephippiger ephippiger. I was particularly interested in whether Mormon crickets synchronize hatching on prime numbered years like periodical cicadas. Cicadas are also underground like Mormon cricket eggs, but unlike Mormon crickets, they are not in diapause and actively feed on tree sap. Cicadas count seasonal cycles, either sensed directly or conveyed through changes in the host, and do not use real time or accumulation of degree days (Karban et al., 2002). Hence, I would expect Mormon crickets to count annual cycles if hatching is synchronized to one or several years (e.g., prime numbered years).

In support of chill time as the environmental cue for embryonic development in Mormon crickets, the proportion of eggs terminating initial diapause increased with the duration of chilling in the Old World tettigoniid Eobiana japonica (Higaki and Ando, 2000). However, Ep. ephippiger was not sensitive to chill time, and one or three months of chilling had the same effect on terminating initial diapause when warmer temperatures were restored (Dean and Hartley, 1977). Chill time is essential for terminating embryonic diapause of fully developed Mormon cricket embryos (Srygley and Senior, 2018), and hence it might also be a cue for termination of initial diapause.

Thirdly, embryonic development of many insects is dependent on a combination of time and temperature above a threshold temperature (i.e., degree days). Ingrisch (1986) found that several tettigoniid katydid species (Tettigonia caudata, Decticus verrucicorus, and Metrioptera saussuriana) had a small proportion of eggs (1–2 %) that reached embryonic diapause after a warm period of 12 weeks at 24 °C, whereas shorter time periods or cooler temperatures resulted in all of the eggs completing embryonic development in the second or as late as the eighth warm period.

Here I focus on differences among sibling groups of Mormon crickets. Prior studies did not account for relatedness of subjects, other than differences among populations. Within a population, the time required for a group of siblings derived from a common mother and father to develop might differ from another group of siblings because of both environmental effects on the parents (e.g., Srygley, 2020a, Srygley, 2020b) and genetic differences. Hence, the study was conducted on multiple groups of siblings within a population to provide replication.