Dale Fort Blog Number 15

23 01 2013


This Blog looks at what could well be the most dangerous animal in the world:


The Mosquitomosquitoe


Mosca is the 16th Century Spanish for a fly (from the Latin Musca).  Mosquito is Spanish for little fly.


The problem with sharing our accommodation with these animals is that since the beginning of humanity they have been responsible for the deaths of millions and millions of us.  They continue to facilitate the deaths of millions of people every year.


Flies can only consume liquid food and both male and female mosquitoes feed on nectar.  Difficulties  arise when the female has to produce eggs.  She needs extra protein for this and it must be in liquid-form.  Her mouth-parts are designed for such occasions and can penetrate the skin of a nearby mammal and suck up high-protein blood from the vessels beneath.


The mammal being pierced tends not to notice because the fly injects a mild anaesthetic and an anti-coagulant prior to penetrating deeper.  You only notice you’ve donated some blood by the immune reaction which develops some time afterwards (itching and swelling).  This blood-sucking habit (haemophagy) causes the transmission and spread of many hideous diseases.


It is estimated that 70 million people become infected with something as a result of mosquito “bites” (actually “sucks”) every year.  One of the worst examples (for humans) is afforded by the Anopheles mosquito.  This carries the single celled parasite Plasmodium  which is responsible for malaria.  The parasite infests the liver and red blood cells and results at best in multiple bouts of debilitating fever and at worst death.  As with all diseases the old and weak are affected most and in 2007 more than 5 million children were estimated to have been killed by malaria.


Yellow fever is also transmitted by mosquitoes as is dengue fever and many other viral diseases.  All in all, it’s a good idea to avoid being pierced.  Having said that, in a temperate climate you are unlikely to get malaria.  Malaria was rife in the Netherlands and around boggy low-lying parts of the UK up until the 19th Century.  The reasons for its decline are not entirely clear but vastly improved sanitation, drainage, sewage treatment and land reclamation are almost certainly important.


The larvae are water creatures and develop rapidly in still water (I found huge numbers in a bucket of rainwater earlier this year).  You can keep them away from you by using a suitable net to sleep under and/or  repellents.  Since they are not strong flyers, a punkawallah or electric fan can be enough to deter them.  The more violently inclined may employ a rolled up newspaper or similar weapon.  This approach is undoubtedly effective but you might have to stay up all night despatching them.


They definitely attack some people more than others,  seeking out their prey by detecting carbon dioxide in exhaled breath.  I am especially attractive to female mosquitoes and have on occasion been left drained, a mere husk of a man by their attentions.  My wife recumbent beside me is left alone with no bites and all her blood.

Look out for the next blog which will denounce the theory of chromatic adaptation (you wouldn’t want to miss that would you?).



16 01 2013

Dale Fort Blog Contents

Number 1

All about nematodes


Number 2

3 You Tube clips:

Starlings at Mabesgate

 Error Bars in Excel 2007

 Measuring Heights on Seashores


Number 3

The History of Dale Fort part 1 (all about the rocks)


Number 4

The History of Dale Fort part 2  (the construction  materials of Dale Fort).  Far more exciting than it sounds, you won’t want to miss it, go there NOW


Number 5

Sargassum muticum in Britain (with a video on how it makes babies)


Number 6

The History of Dale Fort part 3, The First Humans


Number 7

Silverfish and their ways


Number 8

The fat-bellied book chewer


Number 9

Seaweed research at Dale Fort


Number 10

Wormhole research at Dale Fort


Number 11

Limpets and their mysterious ways


Number 12

Anne, Bridget, Cadoc and David


Number 13

St David and his friend Elvis


Number 14

Dancing bananas:  Just how many are there?


Number 15

Six-legged female vampires


Number 16

Cry Havoc!  And let loose the dogs of accountancy………The History of Dale Fort part 6


Number 17

Wee timorous beasties


Number 18

A magical island where strange events take place


Dale Fort Blog Number 14

16 01 2013

This article concerns itself with the problem of how to estimate the size of a population when you can’t count them directly





The Lincoln Index

P = AN/r

P = Population    A = Total in the first sample (i.e. marked)  

N = Total in the second sample      r = Number recaptured


As I mentioned in the video, there are several assumptions that the Lincoln Index makes about the population and what you do to it.  Here they are:

1) The size of the population doesn’t change during the experiment. 

What might happen if it did?  If there were a lot of births (or immigration which is much the same in terms of population numbers) what would that do to the estimate?

Answer: You’ll be diluting the marked animals among more unmarked individuals so the estimate will go up

If there were a lot of deaths (or emigration which is much the same in terms of population numbers) what would that do to the estimate?

Answer: Assuming the marked animals die or emigrate at the same rate as the unmarked, the ratio of marked to unmarked will stay the same.  So your estimate will remain unchanged even though the population has actually gone down (an over-estimate).

The key is to work out what the assumptions are likely to do to the number of recaptures and hence the population estimate.

2) The mark should last for the duration of the experiment

If it doesn’t, what does that do to the number of recaptures?

Answer:  It reduces it leading to an over-estimate

What if wet paint or dye rubbed off on to unmarked animals?

3) The marking procedure should not harm the animals

If the animals were impeded by the procedure are they going to mix up OK?  If the marking material was toxic are they going to feel like socialising with their chums?  What about predators?  Will a predator find the marked animals more easily, or might the mark put a predator off?

4) The population has to have boundaries and inhabit an area which you can define

This might be OK if you’ve got a small pond with some goldfish but it’s likely to be problematical in lots of situations.  How could you define the boundaries of a woodlouse population for example?

5) The individuals in the population (including the marked ones) should mix at random

Apart from drunken people staggering out the pub what population mixes at random?  Motile creatures are going to move in response to stimulation e.g. towards food or shade or moisture.

I think this is one of the biggest problems with mark/recapture.

6) There should be no trap-happy or trap-shy individuals in the population

Often this technique is used in small mammal surveys.  To catch and mark a small mammal you use a baited trap with nesting material in it (a Longworth trap, look it up).  Mammals are good at learning and some quickly realise that getting trapped means free bed and breakfast, no predators and the enticing prospect of escaping into the huge beard of the bloke who opens the trap and there building a cosy nest.  Such individuals are called trap-happy because they actively seek out traps.  The opposite extreme is where individuals find the prospect of being trapped and meeting the ecologist with his vile smelly beard appalling.  These individuals actively avoid the traps and are known as trap-shy animals.  If you have individuals of either persuasion (or both) in the population then your estimate is unlikely to be correct.

Other Points

Sample size

It’s reckoned that you should sample until you get 10% of the marked animals back in the second sample.  So, if you marked 50 in the first sample you should keep sampling in the second sample until you get at least 5 recaptures.

Why do this if it’s got so many problems?

The Lincoln Index is loved by A-Level Exam setters because it allows them to give you a simple calculation to do and then ask you what might have gone wrong.  It’s worth knowing about because the basic idea is easy and it can score you marks.

The Blog will return when I get time to write another one.