# Upper Size Limits for Insects? **URL:** \ **Category:** Factual Questions\ **Created:** [February 21, 2001, 1:55pm UTC](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340 "2001-02-21T13:55:16Z")\ **Posts on this page:** 5\ **Page:** 4
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [July 2, 2002, 6:00am UTC](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340/61 "2002-07-02T06:00:57Z")
If **Duck** and **CRorex** are talking about the same fossil, I find the difference in **CRo** size guesstimates and the actual measurements interesting. Visual guesstimate: 36-42 inches long by 12 inches across Measured size: 16 inches long by 4 inches across (with approx. 20 inch leg-span) This is entirely consistent with my experience with spiders. My visual estimates of size are _always_ 2 or 3 times actual size. However, the actual measurements are still plenty big enough to give me the horrors. Another cozy little spider fact: spider blood pressure is close to that of a human, and they have open circulatory systems. That means that if you punctured a big one, it wouldn’t just ooze nasty bluish ichor…it would _spurt_. Pleasant dreams! 😃 ---
**Author:** ![wolfman](https://avatars.discourse-cdn.com/v4/letter/w/a8b319/32.png) [@wolfman](https://boards.straightdope.com/u/wolfman)\ **Post date:** [July 2, 2002, 6:28am UTC](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340/62 "2002-07-02T06:28:35Z")
> [@](#): > > As an exercise, let’s engineer an elephant-sized roach and see what we get. I just wanted to call first dibs if your experiment fails and you get roach-sized elephants. I would love to have a tank full of itty-bitty elephants. ---
**Author:** ![violet9](https://avatars.discourse-cdn.com/v4/letter/v/ea666f/32.png) [@violet9](https://boards.straightdope.com/u/violet9)\ **Post date:** [July 2, 2002, 9:40pm UTC](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340/63 "2002-07-02T21:40:26Z")
Upper size limit of insects? — GOD, I hope so! ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 3, 2002, 4:52pm UTC](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340/64 "2002-07-03T16:52:24Z")
I was in my backyard yesterday, examing a particular insect, and thinking about this thread, when it hit me. Instead of elephant-sized cockroaches, make elephant-sized fireflys. _That_ would be worth seeing. ---
**Author:** ![easy\_e](https://avatars.discourse-cdn.com/v4/letter/e/eb8c5e/32.png) [@easy\_e](https://boards.straightdope.com/u/easy_e)\ **Post date:** [July 3, 2002, 5:10pm UTC](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340/65 "2002-07-03T17:10:14Z")
> [@](#): > > \*Originally posted by sagitta \* > \*\*Second, I disagree with everyone else (including the experts) about the efficiency of a tracheal system. Large insects pump air in one direction through their tracheae, which permeate their tissues like vertebrates’ capillaries. Air has about the same oxygen content as oxygenated blood, but it has a much lower viscosity. So a tracheal system should be at least as good as a heart-lung arrangement. Diffusion is probably only a problem in the head (most large insects have relatively small heads, not counting horns and other decoration)./B] We covered this in my biology class. IIRC, it’s not so much the _efficiency_ of the tracheal system, it’s the fact that the spiracles have to be lined with exoskeleton (because if they were just “holes” in the tissue, wouldn’t the cells and stuff start to much together?). As you scale up the size of the insect, the spiracles have to be lined with thicker and thicker chitin (to hold back the tissue). Eventually, it gets to be too thick for air to diffuse across it. [Previous page](https://boards.straightdope.com/t/upper-size-limits-for-insects/55340.md?page=3)