Wikipedia:Reference desk/Archives/Science/2018 August 4

= August 4 =

the process of communication between the insects of same species
excuse me, I am very eager to know how the insects communicate among themselves. again, many of them have antenna. so how it works would you please tell me.


 * Insect antennae are not the same as radio antenna, although they do look similar in some cases. Insect antennae are sensory organs for touch, smell, taste, and some other sensory modalities. I am unaware of any insect species ability to communicate electromagnetically with each-other in the radio frequency range. Insect communication may be chemical (e.g. ants, wasps), acoustic (e.g. crickets, cicadas), visual, tactile, etc; and in many cases antennae are indeed involved. Radio waves are not involved, though :), at least as far as I know. Dr Dima (talk) 06:59, 4 August 2018 (UTC)
 * For more details, you may want to read articles on insect antennae that I already mentioned, as well as articles on animal communication and insect senses and communication. Hope this helps! --Dr Dima (talk) 07:04, 4 August 2018 (UTC)


 * See also Ecology and Evolution of Communication in Social Insects. We also have an article on the Waggle dance used by honey bees. Alansplodge (talk) 12:50, 6 August 2018 (UTC)

turtle id in today's featured image
Today's featured image caption says "A Julia butterfly (Dryas iulia) feeding on the tears of a red-headed Amazon River turtle (Podocnemis erythrocephala) ...". Can someone with appropriate expertise please check the turtle identification? The two turtles on the left may be indeed Podocnemis erythrocephala, but the one on the right - the one with the butterfly on its head - doesn't look like P. erythrocephala to me at all. Maybe P. expansa??? I'm not an expert though, so please don't change the featured image caption until an expert has taken a good look at it. I'll also add a note on the image discussion page. Thanks in advance! --Dr Dima (talk) 06:49, 4 August 2018 (UTC)
 * The commons description does say expansa, not erythrocephala. The erythrocephala ID seems to have been given by at Featured_picture_candidates/A_butterfly_feeding_on_the_tears_of_a_turtle_in_Ecuador (ping). I think we should pull the turtle species name for now; will post at WP:ERRORS soon.  Tigraan Click here to contact me 15:57, 4 August 2018 (UTC)
 * The second turtle - the middle one - has a butterfly on its head as well. 194.174.76.21 (talk) 10:32, 6 August 2018 (UTC) Marco Pagliero Berlin

Thermal contraction of helical spring
Imagine you have a helical spring with thickness $$d$$ and unstretched length $$L$$ at room temperature. Then suppose we take the spring into a cryostat and cool it down to something like 10 K. Potentially neglecting the changes to the spring's mechanical properties (its spring constant or Young's modulus, etc.), will the spring stretch due to thermal expansion effects? My intuition tells me that the spring's arclength around the helix will decrease, thus having a small change in the number of turns, but that the length of the spring itself will not change much. Any thoughts would be greatly appreciated! 130.202.62.210 (talk) 17:17, 4 August 2018 (UTC)
 * Of each little bit of the spring shrinks the same in every direction then the overall spring will just shrink the same in every direction, so you should just end up with a spring that looks exactly the same but smaller. Dmcq (talk) 17:53, 4 August 2018 (UTC)
 * Good explanation. An equally simple question is that if you have a steel donut, when you heat it, does the hole get bigger or smaller? Greglocock (talk) 20:21, 4 August 2018 (UTC)


 * Theory or practice?
 * Helical springs pretty much couldn't be made (for large strains) until the 1960s. Otherwise they're just extremely inhomogeneous, so that simple assumptions of consistent linear shrinkage are very unrealistic. Andy Dingley (talk) 22:43, 4 August 2018 (UTC)
 * The way I heard the classic mechanical engineering question was this: Heat up a doughnut-shaped chunk of metal. Does the hole get bigger or smaller? Now inflate a car-tire inner tube in free air. Does the hole get bigger or smaller? --Guy Macon (talk) 23:11, 4 August 2018 (UTC)
 * Doughnuts aren't helical springs. Andy Dingley (talk) 23:13, 4 August 2018 (UTC)
 * The atom to atom links get longer. Greglocock (talk) 10:42, 5 August 2018 (UTC)
 * I don’t know how to post YouTube videos, but if you search YouTube for ‘New tyres for 257 Squadron’ it shows that the hole for steel gets bigger. Widneymanor (talk) 10:47, 5 August 2018 (UTC)

The OP's intuition about the spring's arclength is correct but the helix diameter changes in proportion, thus the number of turns is unchanged by temperature. To answer Guy Macon's question about a metal doughnut and a car-tire inner, the metal is held in shape by interatomic Van der Waals forces that are 3-dimensional (the hole gets bigger) but the inflated inner is a dual-circular Torus shaped by a thin rubber sheet in 2-dimensional tension. Inflation increases its volume V given by
 * $$V = 2 r^2 \pi^2 R$$

where R is the radius of revolution of the center of revolved circle radius r. Since R is virtually constant, r increases and the hole gets smaller. DroneB (talk) 09:53, 6 August 2018 (UTC)