Harvester ants and horny toads were two of the wildlife features in North Central Texas, when I was on summer visits there, from down in the steamy sub-tropics of the Gulf Coast.
The area around the shield-vocano-like harvester mounds is constantly traversed by thousands of ants in active periods. Harvester mandibles are fearsome, even to a human if one gets on your bare toe. I believe that their approaches are a combination of constant cropping, combined with the continuous deposition of pellets of Texas red dirt, brought up from below.

Fire ants are nasty, depending on how one reacts to the stings. I get white blister-like spots which last a while (and itch like crazy.) It is easy to get lots of stings before you know you are under attack. I got systemic reactions from larger assaults and took diphenhydramine antihistamine. They are an invasives in many areas, with varieties that hitch-hiked here in the pots of imported tropical plants. There are native varieties in parts of South and West Texas.
Baking soda or spirits of ammonia can reduce the impact of the stings

[QUOTE=kladner;502135]<snip> Harvester mandibles are fearsome, even to a human if one gets on your bare toe. I believe that their approaches are a combination of constant cropping, combined with the continuous deposition of pellets of Texas red dirt, brought up from below.
[/QUOTE]Bingo! I finally bothered looking it up: The following page about [url=https://wiki.bugwood.org/HPIPM:Harvester_Ants]Harvester Ants[/url] tells us[quote]Harvester ants construct large, highly visible mounds over the network of below ground tunnels that is used by the colony. The mounds often have incorporated into them small pebbles that the ants have excavated and the vegetation around the edge of the nest is cleared. The mound, surface covering materials and cleared area that prevents shading all help to warm the nest. Most often the main nest entrance is located so it orients to the southwest.[/quote]

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[url=https://phys.org/news/2018-12-supernovae-large-ocean-animals-dawn.html]Researchers consider whether supernovae killed off large ocean animals at dawn of Pleistocene[/url] | PhysOrg
[quote][Lead author Adrian] Melott said recent papers revealing ancient seabed deposits of iron-60 isotopes provided the "slam-dunk" evidence of the timing and distance of supernovae.

"As far back as the mid-1990s, people said, 'Hey, look for iron-60. It's a telltale because there's no other way for it to get to Earth but from a supernova.' Because iron-60 is radioactive, if it was formed with the Earth it would be long gone by now. So, it had to have been rained down on us. There's some debate about whether there was only one supernova really nearby or a whole chain of them. I kind of favor a combo of the two—a big chain with one that was unusually powerful and close. If you look at iron-60 residue, there's a huge spike 2.6 million years ago, but there's excess scattered clear back 10 million years."
...
Whether or not there was one supernova or a series of them, the supernova energy that spread layers of iron-60 all over the world also caused penetrating particles called muons to shower Earth, causing cancers and mutations—especially to larger animals.

"The best description of a muon would be a very heavy electron—but a muon is a couple hundred times more massive than an electron," Melott said. "They're very penetrating. Even normally, there are lots of them passing through us. Nearly all of them pass through harmlessly, yet about one-fifth of our radiation dose comes by muons. But when this wave of cosmic rays hits, multiply those muons by a few hundred. Only a small faction of them will interact in any way, but when the number is so large and their energy so high, you get increased mutations and cancer—these would be the main biological effects. We estimated the cancer rate would go up about 50 percent for something the size of a human—and the bigger you are, the worse it is. For an elephant or a whale, the radiation dose goes way up."

A supernova 2.6 million years ago may be related to a marine megafaunal extinction at the Pliocene-Pleistocene boundary where 36 percent of the genera were estimated to become extinct. The extinction was concentrated in coastal waters, where larger organisms would catch a greater radiation dose from the muons.[/quote]
[url=https://en.wikipedia.org/wiki/Iron]Wikipedia on Iron-60[/url]:
[quote][sup]60[/sup]Fe is an extinct radionuclide of long half-life (2.6 million years).[19] It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide [sup]60[/sup]Ni.[18] Much of the past work on isotopic composition of iron has focused on the nucleosynthesis of [sup]60[/sup]Fe through studies of meteorites and ore formation. In the last decade, advances in mass spectrometry have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work is driven by the Earth and planetary science communities, although applications to biological and industrial systems are emerging.[20]

In phases of the meteorites [i]Semarkona[/i] and [i]Chervony Kut[/i], a correlation between the concentration of [sup]60[/sup]Ni, the granddaughter of [sup]60[/sup]Fe, and the abundance of the stable iron isotopes provided evidence for the existence of [sup]60[/sup]Fe at the time of formation of the Solar System. Possibly the energy released by the decay of [sup]60[/sup]Fe, along with that released by [sup]26[/sup]Al, contributed to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of [sup]60[/sup]Ni present in extraterrestrial material may bring further insight into the origin and early history of the Solar System.[21][/quote]

[QUOTE=ewmayer;502569][URL="https://phys.org/news/2018-12-supernovae-large-ocean-animals-dawn.html"]Researchers consider whether supernovae killed off large ocean animals at dawn of Pleistocene[/URL] | PhysOrg

[URL="https://en.wikipedia.org/wiki/Iron"]Wikipedia on Iron-60[/URL]:[/QUOTE]
Lots of interesting follow-on reading to the good first article, from sidebar links. Thanks.

[QUOTE=ewmayer;502569][url=https://phys.org/news/2018-12-supernovae-large-ocean-animals-dawn.html]Researchers consider whether supernovae killed off large ocean animals at dawn of Pleistocene[/url] | PhysOrg

[url=https://en.wikipedia.org/wiki/Iron]Wikipedia on Iron-60[/url]:[/QUOTE]A typical supernova produces a few earth-masses of Al-26, which has a half-life of around 0.7Ma. Four half-lives later the Al-26 should still be detectable. I haven't heard anyone claiming to have found any.

[QUOTE=xilman;502576]A typical supernova produces a few earth-masses of Al-26, which has a half-life of around 0.7Ma. Four half-lives later the Al-26 should still be detectable. I haven't heard anyone claiming to have found any.[/QUOTE] This is a new one on me. <rummage rummage> This isotope is created from spallation of argon by cosmic protons. So there's probably a tiny amount around all the time.

Hmm. Its only decay product is Magnesium-26, which is stable.

I don't know how much supernova-created Al-26 might be expected to have reached Earth -- or how long it would take to get here from a supernova 150 light years away.

[QUOTE=Dr Sardonicus;502654]This is a new one on me. <rummage rummage> This isotope is created from spallation of argon by cosmic protons. So there's probably a tiny amount around all the time.

Hmm. Its only decay product is Magnesium-26, which is stable.

I don't know how much supernova-created Al-26 might be expected to have reached Earth -- or how long it would take to get here from a supernova 150 light years away.[/QUOTE]150ly/2.6My = 1.7 km/s assuming I did the arithmetic correctly. Very slowly, in other words.

Anyway, how did you expect the Fe-60 to get here and not the Al-26 in the same time period?

I was not referring to the Mg-26, but the roughly 6% of the primordial Al-26 which ought to be easily detectable. There are large signals from the Al-26 decay easily detectable towards the galactic centre and out Cygnus way. Why none (or very little) around here?

[QUOTE=xilman;502659]Why none (or very little) around here?[/QUOTE]

strength of gravity ? Also 6.25% over an area of 4*Pi*22500 square light years isn't a lot (~22 ppm on average per square light year)

[QUOTE=xilman;502576]A typical supernova produces a few earth-masses of Al-26, which has a half-life of around 0.7Ma. Four half-lives later the Al-26 should still be detectable. I haven't heard anyone claiming to have found any.[/QUOTE]

Well, a few earth masses may sound like a lot but is really rather tiny compared to the solar-mass-level total ejecta from a SN ... so the key question in that regard is what are the relative mass fractions in the ejecta of 60Fe, 26Al, etc?

Wikipedia gave me no ready answers here, but I found this paper by several CERN scientists which seems pertinent - they do their analysis in the context of the nearby Geminga SN, so their timeframes of greatest interest are a bit different, but the overall approach is quite general: [url=https://cds.cern.ch/record/303599/files/9605128.pdf]Geological Isotope Anomalies As Signatures of Nearby Supernovae[/url]
Key snip (for me):
[quote]There are two ways in which a nearby supernova explosion could produce anomalous isotopes: either indirectly as cosmic ray spallation products, which would be more important for light isotopes such as 10Be, or directly via the deposition of supernova debris, which would be more important for intermediate-mass isotopes such as 41Ca and 60Fe ... The relative importance of these classes of anomalies depends on the distance at which the supernova exploded, since supernova ejecta are slowed down and eventually stopped by the ambient pressure of the interstellar medium (ISM)[/quote]
So "filtering" by way of the ISM provides a mechanism by which heavier isotopes preferentially travel farther from the SN than lighter ones. Any such 'filtering ratio' would need to be applied to the initial mass fractions of 60Fe, 26Al, and for the 2.6Myr timeframe we have a further roughly 10x relative hit to the 26Al fraction due to the difference in decay half-lives. The paper goes on to do various kinds of modeling of "how much stuff makes it way from the SN to earth's surface".

Table 1 at the very end gives expected atomic fractions of various key SN-produced radioisotopes ... indeed, 26Al is expected to give a strong signal. Perhaps the authors of the piece I linked above simply preferred using 60Fe as their marker, due to its much-more-certain association with SN ejecta than 26Al?

[QUOTE=science_man_88;502666]strength of gravity ? Also 6.25% over an area of 4*Pi*22500 square light years isn't a lot (~22 ppm on average per square light year)[/QUOTE]

[B]NOTE[/B]: once you approximate the earth as a perfect sphere of radius 6300 km, and and assume it deposits the ratio of that area to a square light year you can decrease your estimate roughly 16 orders of magnitude further. So to see 1 ppm of earth's
current mass be deposited in the isotope you would need on the order of 2^18 earth masses to begin with.