BBC Wildlife - UK (2019-12)

TheExplainer

T

hestandardprogressionofinsects

fromegg,throughfeedingand

growinglarva,metamorphosing

chrysalis,andfinallytofullywinged

adult,is notquiteuniversal.Batflies

(genusAscodipteron; familyStreblidae)

havetakenthisfurther– orbackwards,

dependingonyourpointofview.Whena

femalebatflyemergesfromherpupaon

thefloorofa batroostina caveorhollow

tree,shematesandfliesuptolocatea

host.Shethenshedsherwings,burrows

underthebat’sskinandstartstorevert

toa maggot-likeform,losingmuch

of the obvious head-thorax-abdomen

segmentationseeninmostadultinsects.

Shefeedsonbatblood,nourishinga

singlelarvainsideherabdomen,ina

bodycavityanalogoustoa womb.When

thelarvais fullygrown,it is released,

dropstotheroostfloorandpupates.

Unlikemostotherinsectlarvaeit does

notfeedorgrowindependently.

Thisis anextremeretrogression,but

severalotherblood-suckingfliesalsolose

theirwingswhentheystartlivingintheir

host’sfur,andqueenantsfamouslyshed

theirwingsaftera matingflightsothey

canfounda newcolony underground.

Richard Jones

Whatis reverse metamorphosis?

Howdoptarmigan

thriveintheArctic?

LIFE-CYCLES

T

herockandwillowptarmigan

aretwoofonly 11 speciesofbird

thatoverwinterinAlaska’sArctic

(alongwiththeredpoll,ravenand

snowyowl).A trick-bagofphysical

andbehaviouraladaptationsallows

thesebirdstoweathertemperaturesas

lowas–40°C,andgainweightwhile

othercreaturesstarve.Inautumn,

theydevelopmorefeathersallover

theirbodies,particularlyonthetoes,

whichprovidewarmthaswellasa

snowshoeeffect(thegenus,Lagopus,

translatesas‘hare-footed’).Theirclaws

alsoincreaseinlength,provinguseful

both as 'crampons' and digging tools.

Ptarmiganscanthus

floatatopthesnow,

probingforburied

catkinsandwillowtwigs

andlaterpeckingatbuds.

Malesstaywhiteinto

spring,whilethefemale

returnstoherdrabplumage,

blendinginwiththenowbrown

landscape.Suchcamouflagehelpsher

toavoidpredationwhilesittingon

hereggs,butthemalehasonelast

trickunderhiswing:enthronedon

a nearbyhillock,heputshimself

inharm’swaytoensuresurvival

of his brood.Michael Engelhard

BIRDS

Starfishandhumansgeta

similar bum deal.

Describedasthesinglemost

importanteventinanyone’slife,

gastrulationis themomentwhen

a veryearly-stageembryo– a

simpleballofcells– foldsinon

itselftoproducea gut,a frontand

backend,andthebasictissue

types.Invertebratessuchas

ourselves,butalsoacornworms

andechinoderms(including

starfish),theinitialdentinthe

ball’ssurfaceeventuallybecomes

theanus;ininsects,molluscsand

almosteverythingelse,it becomes

themouth.Thisfundamental

developmentaldierencepoints

toanancientbranchingofthe

animal family tree. SB

Gastrulation

Ptarmigan: Milo Burcham/Design Pics Inc/Alamy; starfish: Paul Kay/Getty; ant: Richard Becker/FLPA; flounder: Claudio Contreras/naturepl.com

Notquitereverse

metamorphosis,but

a queenant(here

a carpenter)will

shedherwingsfora

subterranean life.

Werefertothewillow

ptarmiganracefoundin

the UK as red grouse.

Q&A

86 BBC Wildlife December 2019