dose of 102,000 R (for 6,000 R/minute) is sufficient to induce a state of lethargy in H. hebetor that is manifested by a complete cessation of activity, including cessation of feeding, and these individuals eventually let themselves starve to death. And an even lower dose of 4,858 R (for 2,650 R/minute) is sufficient to sterilize adult female H. hebetor (sterility arises 3 days post-exposure). Other important factors that influence the level of radioresistance include: The length of time during which a dose of radiation is delivered—with doses delivered during longer periods, or at time intervals, being associated with greatly reduced negative effects; The feeding state of individuals—with pre-fed and post-fed individuals being more resistant to radiations compared to starved individuals; The type of radiation used (e.g., tardigrades Milnesium tardigradum irradiated with heavy ions have a higher survival than when irradiated with gamma rays, for a same irradiation dose); The physiological state of individuals (e.g., the tardigrade species Richtersius coronifer and Milnesium tardigradum are more resistant to gamma-ray radiation when in the hydrated state, and Macrobiotus areolatus is more resistant to X-ray radiation when in the anhydrobiotic state). The way lethality is measured is also source of variation for the estimated radioresistance of a species. Irradiated specimens are not instantly killed, unless exposed to a very high dose (acute dose). Therefore, irradiated specimens die over a certain period of time and lower irradiation doses correspond to longer survival. This means that the radiation dose LD50 fluctuates with the time at which it is measured. For example, the β radiation dose that causes 50% mortality in the American cockroach at 25 days post-exposure is 5,700 R, but to reach 50% mortality at 3 days post-exposure, 45,610 R are needed. 25 days can represent a long survival period for short lived species, such as insects, but would represent a very short survival time for long lived species, such as mammals, so comparing survival of different species after the same amount of time post-exposure also poses some challenges of interpretation. These examples illustrate the many issues associated with comparison of radioresistance among species and the need for caution when doing so. ∗ While an LD50 has been reported for wild type C. elegans individuals, an upper lethal limit has not been established, rather "nearly all animals were alive with no indication of excess lethality up to 800 Gy, the highest dose... measured." See also